Chapter 16. System and Administrative Commands

The startup and shutdown scripts in /etc/rc.d illustrate the uses (and usefulness) of many of these comands. These are usually invoked by root and used for system maintenance or emergency filesystem repairs. Use with caution, as some of these commands may damage your system if misused.

Users and Groups

users

Show all logged on users. This is the approximate equivalent of who -q.

groups

Lists the current user and the groups she belongs to. This corresponds to the $GROUPS internal variable, but gives the group names, rather than the numbers.

 bash$ groups
 bozita cdrom cdwriter audio xgrp
 
 bash$ echo $GROUPS
 501
chown, chgrp

The chown command changes the ownership of a file or files. This command is a useful method that root can use to shift file ownership from one user to another. An ordinary user may not change the ownership of files, not even her own files. [1]

 root# chown bozo *.txt
 
 	      

The chgrp command changes the group ownership of a file or files. You must be owner of the file(s) as well as a member of the destination group (or root) to use this operation.
   1 chgrp --recursive dunderheads *.data
   2 #  The "dunderheads" group will now own all the "*.data" files
   3 #+ all the way down the $PWD directory tree (that's what "recursive" means).

useradd, userdel

The useradd administrative command adds a user account to the system and creates a home directory for that particular user, if so specified. The corresponding userdel command removes a user account from the system [2] and deletes associated files.

Note

The adduser command is a synonym for useradd and is usually a symbolic link to it.

usermod

Modify a user account. Changes may be made to the password, group membership, expiration date, and other attributes of a given user's account. With this command, a user's password may be locked, which has the effect of disabling the account.

groupmod

Modify a given group. The group name and/or ID number may be changed using this command.

id

The id command lists the real and effective user IDs and the group IDs of the user associated with the current process. This is the counterpart to the $UID, $EUID, and $GROUPS internal Bash variables.

 bash$ id
 uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio)
 
 bash$ echo $UID
 501
Note

The id command shows the effective IDs only when they differ from the real ones.

Also see Example 9-5.

lid

The lid (list ID) command shows the group(s) that a given user belongs to, or alternately, the users belonging to a given group. May be invoked only by root.

 root# lid bozo
  bozo(gid=500)
 
 
 root# lid daemon
  bin(gid=1)
  daemon(gid=2)
  adm(gid=4)
  lp(gid=7)
 	      

who

Show all users logged on to the system.

 bash$ who
 bozo  tty1     Apr 27 17:45
 bozo  pts/0    Apr 27 17:46
 bozo  pts/1    Apr 27 17:47
 bozo  pts/2    Apr 27 17:49
 	      

The -m gives detailed information about only the current user. Passing any two arguments to who is the equivalent of who -m, as in who am i or who The Man.

 bash$ who -m
 localhost.localdomain!bozo  pts/2    Apr 27 17:49
 	      

whoami is similar to who -m, but only lists the user name.

 bash$ whoami
 bozo
 	      

w

Show all logged on users and the processes belonging to them. This is an extended version of who. The output of w may be piped to grep to find a specific user and/or process.

 bash$ w | grep startx
 bozo  tty1     -                 4:22pm  6:41   4.47s  0.45s  startx
logname

Show current user's login name (as found in /var/run/utmp). This is a near-equivalent to whoami, above.

 bash$ logname
 bozo
 
 bash$ whoami
 bozo

However . . .

 bash$ su
 Password: ......
 
 bash# whoami
 root
 bash# logname
 bozo

Note

While logname prints the name of the logged in user, whoami gives the name of the user attached to the current process. As we have just seen, sometimes these are not the same.

su

Runs a program or script as a substitute user. su rjones starts a shell as user rjones. A naked su defaults to root. See Example A-15.

sudo

Runs a command as root (or another user). This may be used in a script, thus permitting a regular user to run the script.

   1 #!/bin/bash
   2 
   3 # Some commands.
   4 sudo cp /root/secretfile /home/bozo/secret
   5 # Some more commands.

The file /etc/sudoers holds the names of users permitted to invoke sudo.

passwd

Sets, changes, or manages a user's password.

The passwd command can be used in a script, but probably should not be.


Example 16-1. Setting a new password

   1 #!/bin/bash
   2 #  setnew-password.sh: For demonstration purposes only.
   3 #                      Not a good idea to actually run this script.
   4 #  This script must be run as root.
   5 
   6 ROOT_UID=0         # Root has $UID 0.
   7 E_WRONG_USER=65    # Not root?
   8 
   9 E_NOSUCHUSER=70
  10 SUCCESS=0
  11 
  12 
  13 if [ "$UID" -ne "$ROOT_UID" ]
  14 then
  15   echo; echo "Only root can run this script."; echo
  16   exit $E_WRONG_USER
  17 else
  18   echo
  19   echo "You should know better than to run this script, root."
  20   echo "Even root users get the blues... "
  21   echo
  22 fi  
  23 
  24 
  25 username=bozo
  26 NEWPASSWORD=security_violation
  27 
  28 # Check if bozo lives here.
  29 grep -q "$username" /etc/passwd
  30 if [ $? -ne $SUCCESS ]
  31 then
  32   echo "User $username does not exist."
  33   echo "No password changed."
  34   exit $E_NOSUCHUSER
  35 fi  
  36 
  37 echo "$NEWPASSWORD" | passwd --stdin "$username"
  38 #  The '--stdin' option to 'passwd' permits
  39 #+ getting a new password from stdin (or a pipe).
  40 
  41 echo; echo "User $username's password changed!"
  42 
  43 # Using the 'passwd' command in a script is dangerous.
  44 
  45 exit 0

The passwd command's -l, -u, and -d options permit locking, unlocking, and deleting a user's password. Only root may use these options.

ac

Show users' logged in time, as read from /var/log/wtmp. This is one of the GNU accounting utilities.

 bash$ ac
         total       68.08
last

List last logged in users, as read from /var/log/wtmp. This command can also show remote logins.

For example, to show the last few times the system rebooted:

 bash$ last reboot
 reboot   system boot  2.6.9-1.667      Fri Feb  4 18:18          (00:02)    
 reboot   system boot  2.6.9-1.667      Fri Feb  4 15:20          (01:27)    
 reboot   system boot  2.6.9-1.667      Fri Feb  4 12:56          (00:49)    
 reboot   system boot  2.6.9-1.667      Thu Feb  3 21:08          (02:17)    
 . . .

 wtmp begins Tue Feb  1 12:50:09 2005
newgrp

Change user's group ID without logging out. This permits access to the new group's files. Since users may be members of multiple groups simultaneously, this command finds only limited use.

Note

Kurt Glaesemann points out that the newgrp command could prove helpful in setting the default group permissions for files a user writes. However, the chgrp command might be more convenient for this purpose.

Terminals

tty

Echoes the name (filename) of the current user's terminal. Note that each separate xterm window counts as a different terminal.

 bash$ tty
 /dev/pts/1
stty

Shows and/or changes terminal settings. This complex command, used in a script, can control terminal behavior and the way output displays. See the info page, and study it carefully.


Example 16-2. Setting an erase character

   1 #!/bin/bash
   2 # erase.sh: Using "stty" to set an erase character when reading input.
   3 
   4 echo -n "What is your name? "
   5 read name                      #  Try to backspace
   6                                #+ to erase characters of input.
   7                                #  Problems?
   8 echo "Your name is $name."
   9 
  10 stty erase '#'                 #  Set "hashmark" (#) as erase character.
  11 echo -n "What is your name? "
  12 read name                      #  Use # to erase last character typed.
  13 echo "Your name is $name."
  14 
  15 exit 0
  16 
  17 # Even after the script exits, the new key value remains set.
  18 # Exercise: How would you reset the erase character to the default value?


Example 16-3. secret password: Turning off terminal echoing

   1 #!/bin/bash
   2 # secret-pw.sh: secret password
   3 
   4 echo
   5 echo -n "Enter password "
   6 read passwd
   7 echo "password is $passwd"
   8 echo -n "If someone had been looking over your shoulder, "
   9 echo "your password would have been compromised."
  10 
  11 echo && echo  # Two line-feeds in an "and list."
  12 
  13 
  14 stty -echo    # Turns off screen echo.
  15 
  16 echo -n "Enter password again "
  17 read passwd
  18 echo
  19 echo "password is $passwd"
  20 echo
  21 
  22 stty echo     # Restores screen echo.
  23 
  24 exit 0
  25 
  26 # Do an 'info stty' for more on this useful-but-tricky command.

A creative use of stty is detecting a user keypress (without hitting ENTER).


Example 16-4. Keypress detection

   1 #!/bin/bash
   2 # keypress.sh: Detect a user keypress ("hot keys").
   3 
   4 echo
   5 
   6 old_tty_settings=$(stty -g)   # Save old settings (why?).
   7 stty -icanon
   8 Keypress=$(head -c1)          # or $(dd bs=1 count=1 2> /dev/null)
   9                               # on non-GNU systems
  10 
  11 echo
  12 echo "Key pressed was \""$Keypress"\"."
  13 echo
  14 
  15 stty "$old_tty_settings"      # Restore old settings.
  16 
  17 # Thanks, Stephane Chazelas.
  18 
  19 exit 0

Also see Example 9-3 and Example A-45.

setterm

Set certain terminal attributes. This command writes to its terminal's stdout a string that changes the behavior of that terminal.

 bash$ setterm -cursor off
 bash$
 	      

The setterm command can be used within a script to change the appearance of text written to stdout, although there are certainly better tools available for this purpose.

   1 setterm -bold on
   2 echo bold hello
   3 
   4 setterm -bold off
   5 echo normal hello

tset

Show or initialize terminal settings. This is a less capable version of stty.

 bash$ tset -r
 Terminal type is xterm-xfree86.
 Kill is control-U (^U).
 Interrupt is control-C (^C).
 	      

setserial

Set or display serial port parameters. This command must be run by root and is usually found in a system setup script.

   1 # From /etc/pcmcia/serial script:
   2 
   3 IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'`
   4 setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ

getty, agetty

The initialization process for a terminal uses getty or agetty to set it up for login by a user. These commands are not used within user shell scripts. Their scripting counterpart is stty.

mesg

Enables or disables write access to the current user's terminal. Disabling access would prevent another user on the network to write to the terminal.

Tip

It can be quite annoying to have a message about ordering pizza suddenly appear in the middle of the text file you are editing. On a multi-user network, you might therefore wish to disable write access to your terminal when you need to avoid interruptions.

wall

This is an acronym for "write all," i.e., sending a message to all users at every terminal logged into the network. It is primarily a system administrator's tool, useful, for example, when warning everyone that the system will shortly go down due to a problem (see Example 18-1).

 bash$ wall System going down for maintenance in 5 minutes!
 Broadcast message from bozo (pts/1) Sun Jul  8 13:53:27 2001...

 System going down for maintenance in 5 minutes!
 	      

Note

If write access to a particular terminal has been disabled with mesg, then wall cannot send a message to that terminal.

Information and Statistics

uname

Output system specifications (OS, kernel version, etc.) to stdout. Invoked with the -a option, gives verbose system info (see Example 15-5). The -s option shows only the OS type.

 bash$ uname
 Linux
 
 bash$ uname -s
 Linux
 
 
 bash$ uname -a
 Linux iron.bozo 2.6.15-1.2054_FC5 #1 Tue Mar 14 15:48:33 EST 2006
 i686 i686 i386 GNU/Linux
arch

Show system architecture. Equivalent to uname -m. See Example 10-26.

 bash$ arch
 i686
 
 bash$ uname -m
 i686
lastcomm

Gives information about previous commands, as stored in the /var/account/pacct file. Command name and user name can be specified by options. This is one of the GNU accounting utilities.

lastlog

List the last login time of all system users. This references the /var/log/lastlog file.

 bash$ lastlog
 root          tty1                      Fri Dec  7 18:43:21 -0700 2001
 bin                                     **Never logged in**
 daemon                                  **Never logged in**
 ...
 bozo          tty1                      Sat Dec  8 21:14:29 -0700 2001
 
 
 
 bash$ lastlog | grep root
 root          tty1                      Fri Dec  7 18:43:21 -0700 2001
 	      

Caution

This command will fail if the user invoking it does not have read permission for the /var/log/lastlog file.

lsof

List open files. This command outputs a detailed table of all currently open files and gives information about their owner, size, the processes associated with them, and more. Of course, lsof may be piped to grep and/or awk to parse and analyze its results.

 bash$ lsof
 COMMAND    PID    USER   FD   TYPE     DEVICE    SIZE     NODE NAME
 init         1    root  mem    REG        3,5   30748    30303 /sbin/init
 init         1    root  mem    REG        3,5   73120     8069 /lib/ld-2.1.3.so
 init         1    root  mem    REG        3,5  931668     8075 /lib/libc-2.1.3.so
 cardmgr    213    root  mem    REG        3,5   36956    30357 /sbin/cardmgr
 ...
 	      

The lsof command is a useful, if complex administrative tool. If you are unable to dismount a filesystem and get an error message that it is still in use, then running lsof helps determine which files are still open on that filesystem. The -i option lists open network socket files, and this can help trace intrusion or hack attempts.

 bash$ lsof -an -i tcp
 COMMAND  PID USER  FD  TYPE DEVICE SIZE NODE NAME
 firefox 2330 bozo  32u IPv4   9956       TCP 66.0.118.137:57596->67.112.7.104:http ...
 firefox 2330 bozo  38u IPv4  10535       TCP 66.0.118.137:57708->216.79.48.24:http ...
 	      

strace

System trace: diagnostic and debugging tool for tracing system calls and signals. This command and ltrace, following, are useful for diagnosing why a given program or package fails to run . . . perhaps due to missing libraries or related causes.

 bash$ strace df
 execve("/bin/df", ["df"], [/* 45 vars */]) = 0
 uname({sys="Linux", node="bozo.localdomain", ...}) = 0
 brk(0)                                  = 0x804f5e4

 ...
 	    

This is the Linux equivalent of the Solaris truss command.

ltrace

Library trace: diagnostic and debugging tool that traces library calls invoked by a given command.

 bash$ ltrace df
 __libc_start_main(0x804a910, 1, 0xbfb589a4, 0x804fb70, 0x804fb68 <unfinished ...>:
 setlocale(6, "")                                 = "en_US.UTF-8"
bindtextdomain("coreutils", "/usr/share/locale") = "/usr/share/locale"
textdomain("coreutils")                          = "coreutils"
__cxa_atexit(0x804b650, 0, 0, 0x8052bf0, 0xbfb58908) = 0
getenv("DF_BLOCK_SIZE")                          = NULL

 ...
 	    

nmap

Network mapper and port scanner. This command scans a server to locate open ports and the services associated with those ports. It can also report information about packet filters and firewalls. This is an important security tool for locking down a network against hacking attempts.

   1 #!/bin/bash
   2 
   3 SERVER=$HOST                           # localhost.localdomain (127.0.0.1).
   4 PORT_NUMBER=25                         # SMTP port.
   5 
   6 nmap $SERVER | grep -w "$PORT_NUMBER"  # Is that particular port open?
   7 #              grep -w matches whole words only,
   8 #+             so this wouldn't match port 1025, for example.
   9 
  10 exit 0
  11 
  12 # 25/tcp     open        smtp

nc

The nc (netcat) utility is a complete toolkit for connecting to and listening to TCP and UDP ports. It is useful as a diagnostic and testing tool and as a component in simple script-based HTTP clients and servers.

 bash$ nc localhost.localdomain 25
 220 localhost.localdomain ESMTP Sendmail 8.13.1/8.13.1;
 Thu, 31 Mar 2005 15:41:35 -0700


Example 16-5. Checking a remote server for identd

   1 #! /bin/sh
   2 ## Duplicate DaveG's ident-scan thingie using netcat. Oooh, he'll be p*ssed.
   3 ## Args: target port [port port port ...]
   4 ## Hose stdout _and_ stderr together.
   5 ##
   6 ##  Advantages: runs slower than ident-scan, giving remote inetd less cause
   7 ##+ for alarm, and only hits the few known daemon ports you specify.
   8 ##  Disadvantages: requires numeric-only port args, the output sleazitude,
   9 ##+ and won't work for r-services when coming from high source ports.
  10 # Script author: Hobbit <hobbit@avian.org>
  11 # Used in ABS Guide with permission.
  12 
  13 # ---------------------------------------------------
  14 E_BADARGS=65       # Need at least two args.
  15 TWO_WINKS=2        # How long to sleep.
  16 THREE_WINKS=3
  17 IDPORT=113         # Authentication "tap ident" port.
  18 RAND1=999
  19 RAND2=31337
  20 TIMEOUT0=9
  21 TIMEOUT1=8
  22 TIMEOUT2=4
  23 # ---------------------------------------------------
  24 
  25 case "${2}" in
  26   "" ) echo "Need HOST and at least one PORT." ; exit $E_BADARGS ;;
  27 esac
  28 
  29 # Ping 'em once and see if they *are* running identd.
  30 nc -z -w $TIMEOUT0 "$1" $IDPORT || \
  31 { echo "Oops, $1 isn't running identd." ; exit 0 ; }
  32 #  -z scans for listening daemons.
  33 #     -w $TIMEOUT = How long to try to connect.
  34 
  35 # Generate a randomish base port.
  36 RP=`expr $$ % $RAND1 + $RAND2`
  37 
  38 TRG="$1"
  39 shift
  40 
  41 while test "$1" ; do
  42   nc -v -w $TIMEOUT1 -p ${RP} "$TRG" ${1} < /dev/null > /dev/null &
  43   PROC=$!
  44   sleep $THREE_WINKS
  45   echo "${1},${RP}" | nc -w $TIMEOUT2 -r "$TRG" $IDPORT 2>&1
  46   sleep $TWO_WINKS
  47 
  48 # Does this look like a lamer script or what . . . ?
  49 # ABS Guide author comments: "Ain't really all that bad . . .
  50 #+                            kinda clever, actually."
  51 
  52   kill -HUP $PROC
  53   RP=`expr ${RP} + 1`
  54   shift
  55 done
  56 
  57 exit $?
  58 
  59 #  Notes:
  60 #  -----
  61 
  62 #  Try commenting out line 30 and running this script
  63 #+ with "localhost.localdomain 25" as arguments.
  64 
  65 #  For more of Hobbit's 'nc' example scripts,
  66 #+ look in the documentation:
  67 #+ the /usr/share/doc/nc-X.XX/scripts directory.

And, of course, there's Dr. Andrew Tridgell's notorious one-line script in the BitKeeper Affair:
   1 echo clone | nc thunk.org 5000 > e2fsprogs.dat

free

Shows memory and cache usage in tabular form. The output of this command lends itself to parsing, using grep, awk or Perl. The procinfo command shows all the information that free does, and much more.

 bash$ free
                 total       used       free     shared    buffers     cached
   Mem:         30504      28624       1880      15820       1608       16376
   -/+ buffers/cache:      10640      19864
   Swap:        68540       3128      65412

To show unused RAM memory:

 bash$ free | grep Mem | awk '{ print $4 }'
 1880
procinfo

Extract and list information and statistics from the /proc pseudo-filesystem. This gives a very extensive and detailed listing.

 bash$ procinfo | grep Bootup
 Bootup: Wed Mar 21 15:15:50 2001    Load average: 0.04 0.21 0.34 3/47 6829
lsdev

List devices, that is, show installed hardware.

 bash$ lsdev
 Device            DMA   IRQ  I/O Ports
 ------------------------------------------------
 cascade             4     2 
 dma                          0080-008f
 dma1                         0000-001f
 dma2                         00c0-00df
 fpu                          00f0-00ff
 ide0                     14  01f0-01f7 03f6-03f6
 ...
 	      

du

Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified.

 bash$ du -ach
 1.0k    ./wi.sh
 1.0k    ./tst.sh
 1.0k    ./random.file
 6.0k    .
 6.0k    total
df

Shows filesystem usage in tabular form.

 bash$ df
 Filesystem           1k-blocks      Used Available Use% Mounted on
 /dev/hda5               273262     92607    166547  36% /
 /dev/hda8               222525    123951     87085  59% /home
 /dev/hda7              1408796   1075744    261488  80% /usr
dmesg

Lists all system bootup messages to stdout. Handy for debugging and ascertaining which device drivers were installed and which system interrupts in use. The output of dmesg may, of course, be parsed with grep, sed, or awk from within a script.

 bash$ dmesg | grep hda
 Kernel command line: ro root=/dev/hda2
 hda: IBM-DLGA-23080, ATA DISK drive
 hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63
 hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4
 	      

stat

Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files.

 bash$ stat test.cru
   File: "test.cru"
   Size: 49970        Allocated Blocks: 100          Filetype: Regular File
   Mode: (0664/-rw-rw-r--)         Uid: (  501/ bozo)  Gid: (  501/ bozo)
 Device:  3,8   Inode: 18185     Links: 1    
 Access: Sat Jun  2 16:40:24 2001
 Modify: Sat Jun  2 16:40:24 2001
 Change: Sat Jun  2 16:40:24 2001
 	      

If the target file does not exist, stat returns an error message.

 bash$ stat nonexistent-file
 nonexistent-file: No such file or directory
 	      

In a script, you can use stat to extract information about files (and filesystems) and set variables accordingly.

   1 #!/bin/bash
   2 # fileinfo2.sh
   3 
   4 # Per suggestion of Joël Bourquard and . . .
   5 # http://www.linuxquestions.org/questions/showthread.php?t=410766
   6 
   7 
   8 FILENAME=testfile.txt
   9 file_name=$(stat -c%n "$FILENAME")   # Same as "$FILENAME" of course.
  10 file_owner=$(stat -c%U "$FILENAME")
  11 file_size=$(stat -c%s "$FILENAME")
  12 #  Certainly easier than using "ls -l $FILENAME"
  13 #+ and then parsing with sed.
  14 file_inode=$(stat -c%i "$FILENAME")
  15 file_type=$(stat -c%F "$FILENAME")
  16 file_access_rights=$(stat -c%A "$FILENAME")
  17 
  18 echo "File name:          $file_name"
  19 echo "File owner:         $file_owner"
  20 echo "File size:          $file_size"
  21 echo "File inode:         $file_inode"
  22 echo "File type:          $file_type"
  23 echo "File access rights: $file_access_rights"
  24 
  25 exit 0
  26 
  27 sh fileinfo2.sh
  28 
  29 File name:          testfile.txt
  30 File owner:         bozo
  31 File size:          418
  32 File inode:         1730378
  33 File type:          regular file
  34 File access rights: -rw-rw-r--

vmstat

Display virtual memory statistics.

 bash$ vmstat
    procs                      memory    swap          io system         cpu
 r  b  w   swpd   free   buff  cache  si  so    bi    bo   in    cs  us  sy id
 0  0  0      0  11040   2636  38952   0   0    33     7  271    88   8   3 89
 	    

netstat

Show current network statistics and information, such as routing tables and active connections. This utility accesses information in /proc/net (Chapter 27). See Example 27-4.

netstat -r is equivalent to route.

 bash$ netstat
 Active Internet connections (w/o servers)
 Proto Recv-Q Send-Q Local Address           Foreign Address         State      
 Active UNIX domain sockets (w/o servers)
 Proto RefCnt Flags       Type       State         I-Node Path
 unix  11     [ ]         DGRAM                    906    /dev/log
 unix  3      [ ]         STREAM     CONNECTED     4514   /tmp/.X11-unix/X0
 unix  3      [ ]         STREAM     CONNECTED     4513
 . . .
Note

A netstat -lptu shows sockets that are listening to ports, and the associated processes. This can be useful for determining whether a computer has been hacked or compromised.

uptime

Shows how long the system has been running, along with associated statistics.

 bash$ uptime
 10:28pm  up  1:57,  3 users,  load average: 0.17, 0.34, 0.27

Note

A load average of 1 or less indicates that the system handles processes immediately. A load average greater than 1 means that processes are being queued. When the load average gets above 3, then system performance is significantly degraded.

hostname

Lists the system's host name. This command sets the host name in an /etc/rc.d setup script (/etc/rc.d/rc.sysinit or similar). It is equivalent to uname -n, and a counterpart to the $HOSTNAME internal variable.

 bash$ hostname
 localhost.localdomain
 
 bash$ echo $HOSTNAME
 localhost.localdomain

Similar to the hostname command are the domainname, dnsdomainname, nisdomainname, and ypdomainname commands. Use these to display or set the system DNS or NIS/YP domain name. Various options to hostname also perform these functions.

hostid

Echo a 32-bit hexadecimal numerical identifier for the host machine.

 bash$ hostid
 7f0100

Note

This command allegedly fetches a "unique" serial number for a particular system. Certain product registration procedures use this number to brand a particular user license. Unfortunately, hostid only returns the machine network address in hexadecimal, with pairs of bytes transposed.

The network address of a typical non-networked Linux machine, is found in /etc/hosts.

 bash$ cat /etc/hosts
 127.0.0.1               localhost.localdomain localhost

As it happens, transposing the bytes of 127.0.0.1, we get 0.127.1.0, which translates in hex to 007f0100, the exact equivalent of what hostid returns, above. There exist only a few million other Linux machines with this identical hostid.

sar

Invoking sar (System Activity Reporter) gives a very detailed rundown on system statistics. The Santa Cruz Operation ("Old" SCO) released sar as Open Source in June, 1999.

This command is not part of the base Linux distribution, but may be obtained as part of the sysstat utilities package, written by Sebastien Godard.

 bash$ sar
 Linux 2.4.9 (brooks.seringas.fr) 	09/26/03

10:30:00          CPU     %user     %nice   %system   %iowait     %idle
10:40:00          all      2.21     10.90     65.48      0.00     21.41
10:50:00          all      3.36      0.00     72.36      0.00     24.28
11:00:00          all      1.12      0.00     80.77      0.00     18.11
Average:          all      2.23      3.63     72.87      0.00     21.27

14:32:30          LINUX RESTART

15:00:00          CPU     %user     %nice   %system   %iowait     %idle
15:10:00          all      8.59      2.40     17.47      0.00     71.54
15:20:00          all      4.07      1.00     11.95      0.00     82.98
15:30:00          all      0.79      2.94      7.56      0.00     88.71
Average:          all      6.33      1.70     14.71      0.00     77.26
            
readelf

Show information and statistics about a designated elf binary. This is part of the binutils package.

 bash$ readelf -h /bin/bash
 ELF Header:
   Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
   Class:                             ELF32
   Data:                              2's complement, little endian
   Version:                           1 (current)
   OS/ABI:                            UNIX - System V
   ABI Version:                       0
   Type:                              EXEC (Executable file)
   . . .
size

The size [/path/to/binary] command gives the segment sizes of a binary executable or archive file. This is mainly of use to programmers.

 bash$ size /bin/bash
    text    data     bss     dec     hex filename
  495971   22496   17392  535859   82d33 /bin/bash
 	      

System Logs

logger

Appends a user-generated message to the system log (/var/log/messages). You do not have to be root to invoke logger.

   1 logger Experiencing instability in network connection at 23:10, 05/21.
   2 # Now, do a 'tail /var/log/messages'.

By embedding a logger command in a script, it is possible to write debugging information to /var/log/messages.

   1 logger -t $0 -i Logging at line "$LINENO".
   2 # The "-t" option specifies the tag for the logger entry.
   3 # The "-i" option records the process ID.
   4 
   5 # tail /var/log/message
   6 # ...
   7 # Jul  7 20:48:58 localhost ./test.sh[1712]: Logging at line 3.

logrotate

This utility manages the system log files, rotating, compressing, deleting, and/or e-mailing them, as appropriate. This keeps the /var/log from getting cluttered with old log files. Usually cron runs logrotate on a daily basis.

Adding an appropriate entry to /etc/logrotate.conf makes it possible to manage personal log files, as well as system-wide ones.

Note

Stefano Falsetto has created rottlog, which he considers to be an improved version of logrotate.

Job Control

ps

Process Statistics: lists currently executing processes by owner and PID (process ID). This is usually invoked with ax or aux options, and may be piped to grep or sed to search for a specific process (see Example 14-13 and Example 27-3).

 bash$  ps ax | grep sendmail
 295 ?	   S	  0:00 sendmail: accepting connections on port 25

To display system processes in graphical "tree" format: ps afjx or ps ax --forest.

pgrep, pkill

Combining the ps command with grep or kill.

 bash$ ps a | grep mingetty
 2212 tty2     Ss+    0:00 /sbin/mingetty tty2
 2213 tty3     Ss+    0:00 /sbin/mingetty tty3
 2214 tty4     Ss+    0:00 /sbin/mingetty tty4
 2215 tty5     Ss+    0:00 /sbin/mingetty tty5
 2216 tty6     Ss+    0:00 /sbin/mingetty tty6
 4849 pts/2    S+     0:00 grep mingetty
 
 
 bash$ pgrep mingetty
 2212 mingetty
 2213 mingetty
 2214 mingetty
 2215 mingetty
 2216 mingetty
 	      

Compare the action of pkill with killall.

pstree

Lists currently executing processes in "tree" format. The -p option shows the PIDs, as well as the process names.

top

Continuously updated display of most cpu-intensive processes. The -b option displays in text mode, so that the output may be parsed or accessed from a script.

 bash$ top -b
   8:30pm  up 3 min,  3 users,  load average: 0.49, 0.32, 0.13
 45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped
 CPU states: 13.6% user,  7.3% system,  0.0% nice, 78.9% idle
 Mem:    78396K av,   65468K used,   12928K free,       0K shrd,    2352K buff
 Swap:  157208K av,       0K used,  157208K free                   37244K cached

   PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME COMMAND
   848 bozo      17   0   996  996   800 R     5.6  1.2   0:00 top
     1 root       8   0   512  512   444 S     0.0  0.6   0:04 init
     2 root       9   0     0    0     0 SW    0.0  0.0   0:00 keventd
   ...  
 	      

nice

Run a background job with an altered priority. Priorities run from 19 (lowest) to -20 (highest). Only root may set the negative (higher) priorities. Related commands are renice and snice, which change the priority of a running process or processes, and skill, which sends a kill signal to a process or processes.

nohup

Keeps a command running even after user logs off. The command will run as a foreground process unless followed by &. If you use nohup within a script, consider coupling it with a wait to avoid creating an orphan or zombie process.

pidof

Identifies process ID (PID) of a running job. Since job control commands, such as kill and renice act on the PID of a process (not its name), it is sometimes necessary to identify that PID. The pidof command is the approximate counterpart to the $PPID internal variable.

 bash$ pidof xclock
 880
 	      


Example 16-6. pidof helps kill a process

   1 #!/bin/bash
   2 # kill-process.sh
   3 
   4 NOPROCESS=2
   5 
   6 process=xxxyyyzzz  # Use nonexistent process.
   7 # For demo purposes only...
   8 # ... don't want to actually kill any actual process with this script.
   9 #
  10 # If, for example, you wanted to use this script to logoff the Internet,
  11 #     process=pppd
  12 
  13 t=`pidof $process`       # Find pid (process id) of $process.
  14 # The pid is needed by 'kill' (can't 'kill' by program name).
  15 
  16 if [ -z "$t" ]           # If process not present, 'pidof' returns null.
  17 then
  18   echo "Process $process was not running."
  19   echo "Nothing killed."
  20   exit $NOPROCESS
  21 fi  
  22 
  23 kill $t                  # May need 'kill -9' for stubborn process.
  24 
  25 # Need a check here to see if process allowed itself to be killed.
  26 # Perhaps another " t=`pidof $process` " or ...
  27 
  28 
  29 # This entire script could be replaced by
  30 #        kill $(pidof -x process_name)
  31 # or
  32 #        killall process_name
  33 # but it would not be as instructive.
  34 
  35 exit 0

fuser

Identifies the processes (by PID) that are accessing a given file, set of files, or directory. May also be invoked with the -k option, which kills those processes. This has interesting implications for system security, especially in scripts preventing unauthorized users from accessing system services.

 bash$ fuser -u /usr/bin/vim
 /usr/bin/vim:         3207e(bozo)
 
 
 
 bash$ fuser -u /dev/null
 /dev/null:            3009(bozo)  3010(bozo)  3197(bozo)  3199(bozo)
 	      

One important application for fuser is when physically inserting or removing storage media, such as CD ROM disks or USB flash drives. Sometimes trying a umount fails with a device is busy error message. This means that some user(s) and/or process(es) are accessing the device. An fuser -um /dev/device_name will clear up the mystery, so you can kill any relevant processes.

 bash$ umount /mnt/usbdrive
 umount: /mnt/usbdrive: device is busy
 
 
 
 bash$ fuser -um /dev/usbdrive
 /mnt/usbdrive:        1772c(bozo)
 
 bash$ kill -9 1772
 bash$ umount /mnt/usbdrive
 	      

The fuser command, invoked with the -n option identifies the processes accessing a port. This is especially useful in combination with nmap.

 root# nmap localhost.localdomain
 PORT     STATE SERVICE
 25/tcp   open  smtp
 
 
 
 root# fuser -un tcp 25
 25/tcp:               2095(root)
 
 root# ps ax | grep 2095 | grep -v grep
 2095 ?        Ss     0:00 sendmail: accepting connections
 	      

cron

Administrative program scheduler, performing such duties as cleaning up and deleting system log files and updating the slocate database. This is the superuser version of at (although each user may have their own crontab file which can be changed with the crontab command). It runs as a daemon and executes scheduled entries from /etc/crontab.

Note

Some flavors of Linux run crond, Matthew Dillon's version of cron.

Process Control and Booting

init

The init command is the parent of all processes. Called in the final step of a bootup, init determines the runlevel of the system from /etc/inittab. Invoked by its alias telinit, and by root only.

telinit

Symlinked to init, this is a means of changing the system runlevel, usually done for system maintenance or emergency filesystem repairs. Invoked only by root. This command can be dangerous -- be certain you understand it well before using!

runlevel

Shows the current and last runlevel, that is, whether the system is halted (runlevel 0), in single-user mode (1), in multi-user mode (2 or 3), in X Windows (5), or rebooting (6). This command accesses the /var/run/utmp file.

halt, shutdown, reboot

Command set to shut the system down, usually just prior to a power down.

Warning

On some Linux distros, the halt command has 755 permissions, so it can be invoked by a non-root user. A careless halt in a terminal or a script may shut down the system!

service

Starts or stops a system service. The startup scripts in /etc/init.d and /etc/rc.d use this command to start services at bootup.

 root# /sbin/service iptables stop
 Flushing firewall rules:                                   [  OK  ]
 Setting chains to policy ACCEPT: filter                    [  OK  ]
 Unloading iptables modules:                                [  OK  ]
 	      

Network

ifconfig

Network interface configuration and tuning utility.

 bash$ ifconfig -a
 lo        Link encap:Local Loopback
           inet addr:127.0.0.1  Mask:255.0.0.0
           UP LOOPBACK RUNNING  MTU:16436  Metric:1
           RX packets:10 errors:0 dropped:0 overruns:0 frame:0
           TX packets:10 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:0 
           RX bytes:700 (700.0 b)  TX bytes:700 (700.0 b)

The ifconfig command is most often used at bootup to set up the interfaces, or to shut them down when rebooting.

   1 # Code snippets from /etc/rc.d/init.d/network
   2 
   3 # ...
   4 
   5 # Check that networking is up.
   6 [ ${NETWORKING} = "no" ] && exit 0
   7 
   8 [ -x /sbin/ifconfig ] || exit 0
   9 
  10 # ...
  11 
  12 for i in $interfaces ; do
  13   if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then
  14     action "Shutting down interface $i: " ./ifdown $i boot
  15   fi
  16 #  The GNU-specific "-q" option to "grep" means "quiet", i.e.,
  17 #+ producing no output.
  18 #  Redirecting output to /dev/null is therefore not strictly necessary.
  19        
  20 # ...
  21 
  22 echo "Currently active devices:"
  23 echo `/sbin/ifconfig | grep ^[a-z] | awk '{print $1}'`
  24 #                            ^^^^^  should be quoted to prevent globbing.
  25 #  The following also work.
  26 #    echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })'
  27 #    echo $(/sbin/ifconfig | sed -e 's/ .*//')
  28 #  Thanks, S.C., for additional comments.

See also Example 29-6.

iwconfig

This is the command set for configuring a wireless network. It is the wireless equivalent of ifconfig, above.

ip

General purpose utility for setting up, changing, and analyzing IP (Internet Protocol) networks and attached devices. This command is part of the iproute2 package.

 bash$ ip link show
 1: lo: <LOOPBACK,UP> mtu 16436 qdisc noqueue 
     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
 2: eth0: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast qlen 1000
     link/ether 00:d0:59:ce:af:da brd ff:ff:ff:ff:ff:ff
 3: sit0: <NOARP> mtu 1480 qdisc noop 
     link/sit 0.0.0.0 brd 0.0.0.0
 
 
 bash$ ip route list
 169.254.0.0/16 dev lo  scope link
 	      

Or, in a script:

   1 #!/bin/bash
   2 # Script by Juan Nicolas Ruiz
   3 # Used with his kind permission.
   4 
   5 # Setting up (and stopping) a GRE tunnel.
   6 
   7 
   8 # --- start-tunnel.sh ---
   9 
  10 LOCAL_IP="192.168.1.17"
  11 REMOTE_IP="10.0.5.33"
  12 OTHER_IFACE="192.168.0.100"
  13 REMOTE_NET="192.168.3.0/24"
  14 
  15 /sbin/ip tunnel add netb mode gre remote $REMOTE_IP \
  16   local $LOCAL_IP ttl 255
  17 /sbin/ip addr add $OTHER_IFACE dev netb
  18 /sbin/ip link set netb up
  19 /sbin/ip route add $REMOTE_NET dev netb
  20 
  21 exit 0  #############################################
  22 
  23 # --- stop-tunnel.sh ---
  24 
  25 REMOTE_NET="192.168.3.0/24"
  26 
  27 /sbin/ip route del $REMOTE_NET dev netb
  28 /sbin/ip link set netb down
  29 /sbin/ip tunnel del netb
  30 
  31 exit 0

route

Show info about or make changes to the kernel routing table.

 bash$ route
 Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
 pm3-67.bozosisp *               255.255.255.255 UH       40 0          0 ppp0
 127.0.0.0       *               255.0.0.0       U        40 0          0 lo
 default         pm3-67.bozosisp 0.0.0.0         UG       40 0          0 ppp0
 	      

chkconfig

Check network and system configuration. This command lists and manages the network and system services started at bootup in the /etc/rc?.d directory.

Originally a port from IRIX to Red Hat Linux, chkconfig may not be part of the core installation of some Linux flavors.

 bash$ chkconfig --list
 atd             0:off   1:off   2:off   3:on    4:on    5:on    6:off
 rwhod           0:off   1:off   2:off   3:off   4:off   5:off   6:off
 ...
 	      

tcpdump

Network packet "sniffer." This is a tool for analyzing and troubleshooting traffic on a network by dumping packet headers that match specified criteria.

Dump ip packet traffic between hosts bozoville and caduceus:

 bash$ tcpdump ip host bozoville and caduceus
 	      

Of course, the output of tcpdump can be parsed with certain of the previously discussed text processing utilities.

Filesystem

mount

Mount a filesystem, usually on an external device, such as a floppy or CDROM. The file /etc/fstab provides a handy listing of available filesystems, partitions, and devices, including options, that may be automatically or manually mounted. The file /etc/mtab shows the currently mounted filesystems and partitions (including the virtual ones, such as /proc).

mount -a mounts all filesystems and partitions listed in /etc/fstab, except those with a noauto option. At bootup, a startup script in /etc/rc.d (rc.sysinit or something similar) invokes this to get everything mounted.

   1 mount -t iso9660 /dev/cdrom /mnt/cdrom
   2 # Mounts CD ROM. ISO 9660 is a standard CD ROM filesystem.
   3 mount /mnt/cdrom
   4 # Shortcut, if /mnt/cdrom listed in /etc/fstab

The versatile mount command can even mount an ordinary file on a block device, and the file will act as if it were a filesystem. Mount accomplishes that by associating the file with a loopback device. One application of this is to mount and examine an ISO9660 filesystem image before burning it onto a CDR. [3]


Example 16-7. Checking a CD image

   1 # As root...
   2 
   3 mkdir /mnt/cdtest  # Prepare a mount point, if not already there.
   4 
   5 mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest   # Mount the image.
   6 #                  "-o loop" option equivalent to "losetup /dev/loop0"
   7 cd /mnt/cdtest     # Now, check the image.
   8 ls -alR            # List the files in the directory tree there.
   9                    # And so forth.

umount

Unmount a currently mounted filesystem. Before physically removing a previously mounted floppy or CDROM disk, the device must be umounted, else filesystem corruption may result.
   1 umount /mnt/cdrom
   2 # You may now press the eject button and safely remove the disk.

Note

The automount utility, if properly installed, can mount and unmount floppies or CDROM disks as they are accessed or removed. On "multispindle" laptops with swappable floppy and optical drives, this can cause problems, however.

gnome-mount

The newer Linux distros have deprecated mount and umount. The successor, for command-line mounting of removable storage devices, is gnome-mount. It can take the -d option to mount a device file by its listing in /dev.

For example, to mount a USB flash drive:

 bash$ gnome-mount -d /dev/sda1
 gnome-mount 0.4
 
 
 bash$ df
 . . .
 /dev/sda1                63584     12034     51550  19% /media/disk
  

sync

Forces an immediate write of all updated data from buffers to hard drive (synchronize drive with buffers). While not strictly necessary, a sync assures the sys admin or user that the data just changed will survive a sudden power failure. In the olden days, a sync; sync (twice, just to make absolutely sure) was a useful precautionary measure before a system reboot.

At times, you may wish to force an immediate buffer flush, as when securely deleting a file (see Example 15-60) or when the lights begin to flicker.

losetup

Sets up and configures loopback devices.


Example 16-8. Creating a filesystem in a file

   1 SIZE=1000000  # 1 meg
   2 
   3 head -c $SIZE < /dev/zero > file  # Set up file of designated size.
   4 losetup /dev/loop0 file           # Set it up as loopback device.
   5 mke2fs /dev/loop0                 # Create filesystem.
   6 mount -o loop /dev/loop0 /mnt     # Mount it.
   7 
   8 # Thanks, S.C.

mkswap

Creates a swap partition or file. The swap area must subsequently be enabled with swapon.

swapon, swapoff

Enable / disable swap partitition or file. These commands usually take effect at bootup and shutdown.

mke2fs

Create a Linux ext2 filesystem. This command must be invoked as root.


Example 16-9. Adding a new hard drive

   1 #!/bin/bash
   2 
   3 # Adding a second hard drive to system.
   4 # Software configuration. Assumes hardware already mounted.
   5 # From an article by the author of the ABS Guide.
   6 # In issue #38 of _Linux Gazette_, http://www.linuxgazette.com.
   7 
   8 ROOT_UID=0     # This script must be run as root.
   9 E_NOTROOT=67   # Non-root exit error.
  10 
  11 if [ "$UID" -ne "$ROOT_UID" ]
  12 then
  13   echo "Must be root to run this script."
  14   exit $E_NOTROOT
  15 fi  
  16 
  17 # Use with extreme caution!
  18 # If something goes wrong, you may wipe out your current filesystem.
  19 
  20 
  21 NEWDISK=/dev/hdb         # Assumes /dev/hdb vacant. Check!
  22 MOUNTPOINT=/mnt/newdisk  # Or choose another mount point.
  23 
  24 
  25 fdisk $NEWDISK
  26 mke2fs -cv $NEWDISK1   # Check for bad blocks (verbose output).
  27 #  Note:           ^     /dev/hdb1, *not* /dev/hdb!
  28 mkdir $MOUNTPOINT
  29 chmod 777 $MOUNTPOINT  # Makes new drive accessible to all users.
  30 
  31 
  32 # Now, test ...
  33 # mount -t ext2 /dev/hdb1 /mnt/newdisk
  34 # Try creating a directory.
  35 # If it works, umount it, and proceed.
  36 
  37 # Final step:
  38 # Add the following line to /etc/fstab.
  39 # /dev/hdb1  /mnt/newdisk  ext2  defaults  1 1
  40 
  41 exit

See also Example 16-8 and Example 28-3.

tune2fs

Tune ext2 filesystem. May be used to change filesystem parameters, such as maximum mount count. This must be invoked as root.

Warning

This is an extremely dangerous command. Use it at your own risk, as you may inadvertently destroy your filesystem.

dumpe2fs

Dump (list to stdout) very verbose filesystem info. This must be invoked as root.

 root# dumpe2fs /dev/hda7 | grep 'ount count'
 dumpe2fs 1.19, 13-Jul-2000 for EXT2 FS 0.5b, 95/08/09
 Mount count:              6
 Maximum mount count:      20
hdparm

List or change hard disk parameters. This command must be invoked as root, and it may be dangerous if misused.

fdisk

Create or change a partition table on a storage device, usually a hard drive. This command must be invoked as root.

Warning

Use this command with extreme caution. If something goes wrong, you may destroy an existing filesystem.

fsck, e2fsck, debugfs

Filesystem check, repair, and debug command set.

fsck: a front end for checking a UNIX filesystem (may invoke other utilities). The actual filesystem type generally defaults to ext2.

e2fsck: ext2 filesystem checker.

debugfs: ext2 filesystem debugger. One of the uses of this versatile, but dangerous command is to (attempt to) recover deleted files. For advanced users only!

Caution

All of these should be invoked as root, and they can damage or destroy a filesystem if misused.

badblocks

Checks for bad blocks (physical media flaws) on a storage device. This command finds use when formatting a newly installed hard drive or testing the integrity of backup media. [4] As an example, badblocks /dev/fd0 tests a floppy disk.

The badblocks command may be invoked destructively (overwrite all data) or in non-destructive read-only mode. If root user owns the device to be tested, as is generally the case, then root must invoke this command.

lsusb, usbmodules

The lsusb command lists all USB (Universal Serial Bus) buses and the devices hooked up to them.

The usbmodules command outputs information about the driver modules for connected USB devices.

 bash$ lsusb
 Bus 001 Device 001: ID 0000:0000  
 Device Descriptor:
   bLength                18
   bDescriptorType         1
   bcdUSB               1.00
   bDeviceClass            9 Hub
   bDeviceSubClass         0 
   bDeviceProtocol         0 
   bMaxPacketSize0         8
   idVendor           0x0000 
   idProduct          0x0000

   . . .
 	      

lspci

Lists pci busses present.

 bash$ lspci
 00:00.0 Host bridge: Intel Corporation 82845 845
 (Brookdale) Chipset Host Bridge (rev 04)
 00:01.0 PCI bridge: Intel Corporation 82845 845
 (Brookdale) Chipset AGP Bridge (rev 04)
 00:1d.0 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #1) (rev 02)
 00:1d.1 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #2) (rev 02)
 00:1d.2 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #3) (rev 02)
 00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev 42)

   . . .
 	      

mkbootdisk

Creates a boot floppy which can be used to bring up the system if, for example, the MBR (master boot record) becomes corrupted. Of special interest is the --iso option, which uses mkisofs to create a bootable ISO9660 filesystem image suitable for burning a bootable CDR.

The mkbootdisk command is actually a Bash script, written by Erik Troan, in the /sbin directory.

mkisofs

Creates an ISO9660 filesystem suitable for a CDR image.

chroot

CHange ROOT directory. Normally commands are fetched from $PATH, relative to /, the default root directory. This changes the root directory to a different one (and also changes the working directory to there). This is useful for security purposes, for instance when the system administrator wishes to restrict certain users, such as those telnetting in, to a secured portion of the filesystem (this is sometimes referred to as confining a guest user to a "chroot jail"). Note that after a chroot, the execution path for system binaries is no longer valid.

A chroot /opt would cause references to /usr/bin to be translated to /opt/usr/bin. Likewise, chroot /aaa/bbb /bin/ls would redirect future instances of ls to /aaa/bbb as the base directory, rather than / as is normally the case. An alias XX 'chroot /aaa/bbb ls' in a user's ~/.bashrc effectively restricts which portion of the filesystem she may run command "XX" on.

The chroot command is also handy when running from an emergency boot floppy (chroot to /dev/fd0), or as an option to lilo when recovering from a system crash. Other uses include installation from a different filesystem (an rpm option) or running a readonly filesystem from a CD ROM. Invoke only as root, and use with care.

Caution

It might be necessary to copy certain system files to a chrooted directory, since the normal $PATH can no longer be relied upon.

lockfile

This utility is part of the procmail package (www.procmail.org). It creates a lock file, a semaphore that controls access to a file, device, or resource.

The lock file serves as a flag that this particular file, device, or resource is in use by a process (and is therefore "busy"). The presence of a lock file permits only restricted access (or no access) to other processes.

   1 lockfile /home/bozo/lockfiles/$0.lock
   2 # Creates a write-protected lockfile prefixed with the name of the script.
   3 
   4 lockfile /home/bozo/lockfiles/${0##*/}.lock
   5 # A safer version of the above, as pointed out by E. Choroba.

Lock files are used in such applications as protecting system mail folders from simultaneously being changed by multiple users, indicating that a modem port is being accessed, and showing that an instance of Firefox is using its cache. Scripts may check for the existence of a lock file created by a certain process to check if that process is running. Note that if a script attempts to create a lock file that already exists, the script will likely hang.

Normally, applications create and check for lock files in the /var/lock directory. [5] A script can test for the presence of a lock file by something like the following.
   1 appname=xyzip
   2 # Application "xyzip" created lock file "/var/lock/xyzip.lock".
   3 
   4 if [ -e "/var/lock/$appname.lock" ]
   5 then   #+ Prevent other programs & scripts
   6        #  from accessing files/resources used by xyzip.
   7   ...

flock

Much less useful than the lockfile command is flock. It sets an "advisory" lock on a file and then executes a command while the lock is on. This is to prevent any other process from setting a lock on that file until completion of the specified command.

   1 flock $0 cat $0 > lockfile__$0
   2 #  Set a lock on the script the above line appears in,
   3 #+ while listing the script to stdout.

Note

Unlike lockfile, flock does not automatically create a lock file.

mknod

Creates block or character device files (may be necessary when installing new hardware on the system). The MAKEDEV utility has virtually all of the functionality of mknod, and is easier to use.

MAKEDEV

Utility for creating device files. It must be run as root, and in the /dev directory. It is a sort of advanced version of mknod.

tmpwatch

Automatically deletes files which have not been accessed within a specified period of time. Usually invoked by cron to remove stale log files.

Backup

dump, restore

The dump command is an elaborate filesystem backup utility, generally used on larger installations and networks. [6] It reads raw disk partitions and writes a backup file in a binary format. Files to be backed up may be saved to a variety of storage media, including disks and tape drives. The restore command restores backups made with dump.

fdformat

Perform a low-level format on a floppy disk (/dev/fd0*).

System Resources

ulimit

Sets an upper limit on use of system resources. Usually invoked with the -f option, which sets a limit on file size (ulimit -f 1000 limits files to 1 meg maximum). The -t option limits the coredump size (ulimit -c 0 eliminates coredumps). Normally, the value of ulimit would be set in /etc/profile and/or ~/.bash_profile (see Appendix G).

Important

Judicious use of ulimit can protect a system against the dreaded fork bomb.

   1 #!/bin/bash
   2 # This script is for illustrative purposes only.
   3 # Run it at your own peril -- it WILL freeze your system.
   4 
   5 while true  #  Endless loop.
   6 do
   7   $0 &      #  This script invokes itself . . .
   8             #+ forks an infinite number of times . . .
   9             #+ until the system freezes up because all resources exhausted.
  10 done        #  This is the notorious "sorcerer's appentice" scenario.
  11 
  12 exit 0      #  Will not exit here, because this script will never terminate.

A ulimit -Hu XX (where XX is the user process limit) in /etc/profile would abort this script when it exceeded the preset limit.

quota

Display user or group disk quotas.

setquota

Set user or group disk quotas from the command-line.

umask

User file creation permissions mask. Limit the default file attributes for a particular user. All files created by that user take on the attributes specified by umask. The (octal) value passed to umask defines the file permissions disabled. For example, umask 022 ensures that new files will have at most 755 permissions (777 NAND 022). [7] Of course, the user may later change the attributes of particular files with chmod. The usual practice is to set the value of umask in /etc/profile and/or ~/.bash_profile (see Appendix G).


Example 16-10. Using umask to hide an output file from prying eyes

   1 #!/bin/bash
   2 # rot13a.sh: Same as "rot13.sh" script, but writes output to "secure" file.
   3 
   4 # Usage: ./rot13a.sh filename
   5 # or     ./rot13a.sh <filename
   6 # or     ./rot13a.sh and supply keyboard input (stdin)
   7 
   8 umask 177               #  File creation mask.
   9                         #  Files created by this script
  10                         #+ will have 600 permissions.
  11 
  12 OUTFILE=decrypted.txt   #  Results output to file "decrypted.txt"
  13                         #+ which can only be read/written
  14                         #  by invoker of script (or root).
  15 
  16 cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M' > $OUTFILE 
  17 #    ^^ Input from stdin or a file.   ^^^^^^^^^^ Output redirected to file. 
  18 
  19 exit 0

rdev

Get info about or make changes to root device, swap space, or video mode. The functionality of rdev has generally been taken over by lilo, but rdev remains useful for setting up a ram disk. This is a dangerous command, if misused.

Modules

lsmod

List installed kernel modules.

 bash$ lsmod
 Module                  Size  Used by
 autofs                  9456   2 (autoclean)
 opl3                   11376   0
 serial_cs               5456   0 (unused)
 sb                     34752   0
 uart401                 6384   0 [sb]
 sound                  58368   0 [opl3 sb uart401]
 soundlow                 464   0 [sound]
 soundcore               2800   6 [sb sound]
 ds                      6448   2 [serial_cs]
 i82365                 22928   2
 pcmcia_core            45984   0 [serial_cs ds i82365]
 	      

Note

Doing a cat /proc/modules gives the same information.

insmod

Force installation of a kernel module (use modprobe instead, when possible). Must be invoked as root.

rmmod

Force unloading of a kernel module. Must be invoked as root.

modprobe

Module loader that is normally invoked automatically in a startup script. Must be invoked as root.

depmod

Creates module dependency file. Usually invoked from a startup script.

modinfo

Output information about a loadable module.

 bash$ modinfo hid
 filename:    /lib/modules/2.4.20-6/kernel/drivers/usb/hid.o
 description: "USB HID support drivers"
 author:      "Andreas Gal, Vojtech Pavlik <vojtech@suse.cz>"
 license:     "GPL"
 	      

Miscellaneous

env

Runs a program or script with certain environmental variables set or changed (without changing the overall system environment). The [varname=xxx] permits changing the environmental variable varname for the duration of the script. With no options specified, this command lists all the environmental variable settings. [8]

Note

The first line of a script (the "sha-bang" line) may use env when the path to the shell or interpreter is unknown.

   1 #! /usr/bin/env perl
   2 
   3 print "This Perl script will run,\n";
   4 print "even when I don't know where to find Perl.\n";
   5 
   6 # Good for portable cross-platform scripts,
   7 # where the Perl binaries may not be in the expected place.
   8 # Thanks, S.C.

Or even ...

   1 #!/bin/env bash
   2 # Queries the $PATH enviromental variable for the location of bash.
   3 # Therefore ...
   4 # This script will run where Bash is not in its usual place, in /bin.
   5 ...

ldd

Show shared lib dependencies for an executable file.

 bash$ ldd /bin/ls
 libc.so.6 => /lib/libc.so.6 (0x4000c000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)
watch

Run a command repeatedly, at specified time intervals.

The default is two-second intervals, but this may be changed with the -n option.

   1 watch -n 5 tail /var/log/messages
   2 # Shows tail end of system log, /var/log/messages, every five seconds.

Note

Unfortunately, piping the output of watch command to grep does not work.

strip

Remove the debugging symbolic references from an executable binary. This decreases its size, but makes debugging it impossible.

This command often occurs in a Makefile, but rarely in a shell script.

nm

List symbols in an unstripped compiled binary.

rdist

Remote distribution client: synchronizes, clones, or backs up a file system on a remote server.

16.1. Analyzing a System Script

Using our knowledge of administrative commands, let us examine a system script. One of the shortest and simplest to understand scripts is "killall," [9] used to suspend running processes at system shutdown.


Example 16-11. killall, from /etc/rc.d/init.d

   1 #!/bin/sh
   2 
   3 # --> Comments added by the author of this document marked by "# -->".
   4 
   5 # --> This is part of the 'rc' script package
   6 # --> by Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>.
   7 
   8 # --> This particular script seems to be Red Hat / FC specific
   9 # --> (may not be present in other distributions).
  10 
  11 #  Bring down all unneeded services that are still running
  12 #+ (there shouldn't be any, so this is just a sanity check)
  13 
  14 for i in /var/lock/subsys/*; do
  15         # --> Standard for/in loop, but since "do" is on same line,
  16         # --> it is necessary to add ";".
  17         # Check if the script is there.
  18         [ ! -f $i ] && continue
  19         # --> This is a clever use of an "and list", equivalent to:
  20         # --> if [ ! -f "$i" ]; then continue
  21 
  22         # Get the subsystem name.
  23         subsys=${i#/var/lock/subsys/}
  24         # --> Match variable name, which, in this case, is the file name.
  25         # --> This is the exact equivalent of subsys=`basename $i`.
  26 	
  27         # -->  It gets it from the lock file name
  28         # -->+ (if there is a lock file,
  29         # -->+ that's proof the process has been running).
  30         # -->  See the "lockfile" entry, above.
  31 
  32 
  33         # Bring the subsystem down.
  34         if [ -f /etc/rc.d/init.d/$subsys.init ]; then
  35            /etc/rc.d/init.d/$subsys.init stop
  36         else
  37            /etc/rc.d/init.d/$subsys stop
  38         # -->  Suspend running jobs and daemons.
  39         # -->  Note that "stop" is a positional parameter,
  40         # -->+ not a shell builtin.
  41         fi
  42 done

That wasn't so bad. Aside from a little fancy footwork with variable matching, there is no new material there.

Exercise 1. In /etc/rc.d/init.d, analyze the halt script. It is a bit longer than killall, but similar in concept. Make a copy of this script somewhere in your home directory and experiment with it (do not run it as root). Do a simulated run with the -vn flags (sh -vn scriptname). Add extensive comments. Change the "action" commands to "echos".

Exercise 2. Look at some of the more complex scripts in /etc/rc.d/init.d. See if you can understand parts of them. Follow the above procedure to analyze them. For some additional insight, you might also examine the file sysvinitfiles in /usr/share/doc/initscripts-?.??, which is part of the "initscripts" documentation.

Notes

[1]

This is the case on a Linux machine or a UNIX system with disk quotas.

[2]

The userdel command will fail if the particular user being deleted is still logged on.

[3]

For more detail on burning CDRs, see Alex Withers' article, Creating CDs, in the October, 1999 issue of Linux Journal.

[4]

The -c option to mke2fs also invokes a check for bad blocks.

[5]

Since only root has write permission in the /var/lock directory, a user script cannot set a lock file there.

[6]

Operators of single-user Linux systems generally prefer something simpler for backups, such as tar.

[7]

NAND is the logical not-and operator. Its effect is somewhat similar to subtraction.

[8]

In Bash and other Bourne shell derivatives, it is possible to set variables in a single command's environment.
   1 var1=value1 var2=value2 commandXXX
   2 # $var1 and $var2 set in the environment of 'commandXXX' only.

[9]

The killall system script should not be confused with the killall command in /usr/bin.