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" (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" \> "$b" ]
Note that the ">" needs to be escaped within a [ ] construct.
See Example 26-11 for an application of this comparison operator.
- -n
-
string is not "null."
The -n test absolutely requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7-6) normally works, however, this is an unsafe practice. Always quote a tested string. [1]
- -z
-
string is "null, " that is, has zero length
Example 7-5. Arithmetic and string comparisons
1 #!/bin/bash 2 3 a=4 4 b=5 5 6 # Here "a" and "b" can be treated either as integers or strings. 7 # There is some blurring between the arithmetic and string comparisons, 8 #+ since Bash variables are not strongly typed. 9 10 # Bash permits integer operations and comparisons on variables 11 #+ whose value consists of all-integer characters. 12 # Caution advised, however. 13 14 echo 15 16 if [ "$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
8 # If a string has not been initialized, it has no defined value.
9 # This state is called "null" (not the same as zero).
10
11 if [ -n $string1 ] # $string1 has not been declared or initialized.
12 then
13 echo "String \"string1\" is not null."
14 else
15 echo "String \"string1\" is null."
16 fi
17 # Wrong result.
18 # Shows $string1 as not null, although it was not initialized.
19
20
21 echo
22
23
24 # Lets try it again.
25
26 if [ -n "$string1" ] # This time, $string1 is quoted.
27 then
28 echo "String \"string1\" is not null."
29 else
30 echo "String \"string1\" is null."
31 fi # Quote strings within test brackets!
32
33
34 echo
35
36
37 if [ $string1 ] # This time, $string1 stands naked.
38 then
39 echo "String \"string1\" is not null."
40 else
41 echo "String \"string1\" is null."
42 fi
43 # This works fine.
44 # The [ ] test operator alone detects whether the string is null.
45 # However it is good practice to quote it ("$string1").
46 #
47 # As Stephane Chazelas points out,
48 # if [ $string1 ] has one argument, "]"
49 # if [ "$string1" ] has two arguments, the empty "$string1" and "]"
50
51
52
53 echo
54
55
56
57 string1=initialized
58
59 if [ $string1 ] # Again, $string1 stands naked.
60 then
61 echo "String \"string1\" is not null."
62 else
63 echo "String \"string1\" is null."
64 fi
65 # Again, gives correct result.
66 # Still, it is better to quote it ("$string1"), because . . .
67
68
69 string1="a = b"
70
71 if [ $string1 ] # Again, $string1 stands naked.
72 then
73 echo "String \"string1\" is not null."
74 else
75 echo "String \"string1\" is null."
76 fi
77 # Not quoting "$string1" now gives wrong result!
78
79 exit 0
80 # 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'
5
6 NOARGS=65
7 NOTFOUND=66
8 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 $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
24 # Error message to stderr.
25 exit $NOTFOUND
26 fi
27
28 if [ ${filename##*.} != "gz" ]
29 # Using bracket in variable substitution.
30 then
31 echo "File $1 is not a gzipped file!"
32 exit $NOTGZIP
33 fi
34
35 zcat $1 | more
36
37 # Uses the filter 'more.'
38 # May substitute 'less', if desired.
39
40
41 exit $? # Script returns exit status of pipe.
42 # Actually "exit $?" is unnecessary, as the script will, in any case,
43 # return the exit status of the last command executed. |
These are similar to the Bash comparison operators && and ||, used within double brackets.
1 [[ condition1 && condition2 ]] |
1 if [ "$exp1" -a "$exp2" ] |
Refer to Example 8-3, Example 26-16, and Example A-30 to see compound comparison operators in action.