7.3. Other Comparison Operators
A binary comparison operator compares two variables or quantities. Note that integer and string comparison use a different set of operators.
- -eq
is equal to
if [ "$a" -eq "$b" ]
- -ne
is not equal to
if [ "$a" -ne "$b" ]
- -gt
is greater than
if [ "$a" -gt "$b" ]
- -ge
is greater than or equal to
if [ "$a" -ge "$b" ]
- -lt
is less than
if [ "$a" -lt "$b" ]
- -le
is less than or equal to
if [ "$a" -le "$b" ]
- <
is less than (within double parentheses)
(("$a" < "$b"))
- <=
is less than or equal to (within double parentheses)
(("$a" <= "$b"))
- >
is greater than (within double parentheses)
(("$a" > "$b"))
- >=
is greater than or equal to (within double parentheses)
(("$a" >= "$b"))
- =
is equal to
if [ "$a" = "$b" ]
- ==
is equal to
if [ "$a" == "$b" ]
This is a synonym for =.
The == comparison operator behaves differently within a double-brackets test than within single brackets.
1 [[ $a == z* ]] # True if $a starts with an "z" (regex pattern matching). 2 [[ $a == "z*" ]] # True if $a is equal to z* (literal matching). 3 4 [ $a == z* ] # File globbing and word splitting take place. 5 [ "$a" == "z*" ] # True if $a is equal to z* (literal matching). 6 7 # Thanks, Stéphane Chazelas
- !=
is not equal to
if [ "$a" != "$b" ]
This operator uses pattern matching within a [[ ... ]] construct.
- <
is less than, in ASCII alphabetical order
if [[ "$a" < "$b" ]]
if [ "$a" \< "$b" ]
Note that the "<" needs to be escaped within a [ ] construct.
- >
is greater than, in ASCII alphabetical order
if [[ "$a" > "$b" ]]
if [ "$a" \> "$b" ]
Note that the ">" needs to be escaped within a [ ] construct.
See Example 26-11 for an application of this comparison operator.
- -z
string is null, that is, has zero length
1 String='' # Zero-length ("null") string variable. 2 3 if [ -z "$String" ] 4 then 5 echo "\$String is null." 6 else 7 echo "\$String is NOT null." 8 fi # $String is null.- -n
string is not null.
The -n test requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7-6) normally works, however, this is an unsafe practice. Always quote a tested string. [1]
Example 7-5. Arithmetic and string comparisons
1 #!/bin/bash 2 3 a=4 4 b=5 5 6 # Here "a" and "b" can be treated either as integers or strings. 7 # There is some blurring between the arithmetic and string comparisons, 8 #+ since Bash variables are not strongly typed. 9 10 # Bash permits integer operations and comparisons on variables 11 #+ whose value consists of all-integer characters. 12 # Caution advised, however. 13 14 echo 15 16 if [ "$a" -ne "$b" ] 17 then 18 echo "$a is not equal to $b" 19 echo "(arithmetic comparison)" 20 fi 21 22 echo 23 24 if [ "$a" != "$b" ] 25 then 26 echo "$a is not equal to $b." 27 echo "(string comparison)" 28 # "4" != "5" 29 # ASCII 52 != ASCII 53 30 fi 31 32 # In this particular instance, both "-ne" and "!=" work. 33 34 echo 35 36 exit 0 |
Example 7-6. Testing whether a string is null
1 #!/bin/bash
2 # str-test.sh: Testing null strings and unquoted strings,
3 #+ but not strings and sealing wax, not to mention cabbages and kings . . .
4
5 # Using if [ ... ]
6
7 # If a string has not been initialized, it has no defined value.
8 # This state is called "null" (not the same as zero!).
9
10 if [ -n $string1 ] # string1 has not been declared or initialized.
11 then
12 echo "String \"string1\" is not null."
13 else
14 echo "String \"string1\" is null."
15 fi # Wrong result.
16 # Shows $string1 as not null, although it was not initialized.
17
18 echo
19
20 # Let's try it again.
21
22 if [ -n "$string1" ] # This time, $string1 is quoted.
23 then
24 echo "String \"string1\" is not null."
25 else
26 echo "String \"string1\" is null."
27 fi # Quote strings within test brackets!
28
29 echo
30
31 if [ $string1 ] # This time, $string1 stands naked.
32 then
33 echo "String \"string1\" is not null."
34 else
35 echo "String \"string1\" is null."
36 fi # This works fine.
37 # The [ ... ] test operator alone detects whether the string is null.
38 # However it is good practice to quote it (if [ "$string1" ]).
39 #
40 # As Stephane Chazelas points out,
41 # if [ $string1 ] has one argument, "]"
42 # if [ "$string1" ] has two arguments, the empty "$string1" and "]"
43
44
45 echo
46
47
48 string1=initialized
49
50 if [ $string1 ] # Again, $string1 stands unquoted.
51 then
52 echo "String \"string1\" is not null."
53 else
54 echo "String \"string1\" is null."
55 fi # Again, gives correct result.
56 # Still, it is better to quote it ("$string1"), because . . .
57
58
59 string1="a = b"
60
61 if [ $string1 ] # Again, $string1 stands unquoted.
62 then
63 echo "String \"string1\" is not null."
64 else
65 echo "String \"string1\" is null."
66 fi # Not quoting "$string1" now gives wrong result!
67
68 exit 0 # Thank you, also, Florian Wisser, for the "heads-up". |
Example 7-7. zmore
1 #!/bin/bash
2 # zmore
3
4 # View gzipped files with 'more' filter.
5
6 E_NOARGS=65
7 E_NOTFOUND=66
8 E_NOTGZIP=67
9
10 if [ $# -eq 0 ] # same effect as: if [ -z "$1" ]
11 # $1 can exist, but be empty: zmore "" arg2 arg3
12 then
13 echo "Usage: `basename $0` filename" >&2
14 # Error message to stderr.
15 exit $E_NOARGS
16 # Returns 65 as exit status of script (error code).
17 fi
18
19 filename=$1
20
21 if [ ! -f "$filename" ] # Quoting $filename allows for possible spaces.
22 then
23 echo "File $filename not found!" >&2 # Error message to stderr.
24 exit $E_NOTFOUND
25 fi
26
27 if [ ${filename##*.} != "gz" ]
28 # Using bracket in variable substitution.
29 then
30 echo "File $1 is not a gzipped file!"
31 exit $E_NOTGZIP
32 fi
33
34 zcat $1 | more
35
36 # Uses the 'more' filter.
37 # May substitute 'less' if desired.
38
39 exit $? # Script returns exit status of pipe.
40 # Actually "exit $?" is unnecessary, as the script will, in any case,
41 #+ return the exit status of the last command executed. |
These are similar to the Bash comparison operators && and ||, used within double brackets.
1 [[ condition1 && condition2 ]] |
The -o and -a operators work with the test command or occur within single test brackets.
1 if [ "$exp1" -a "$exp2" ] |
Refer to Example 8-3, Example 26-17, and Example A-31 to see compound comparison operators in action.