7.3. Comparison operators (binary)
- -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 =.
1 [[ $a == z* ]] # true if $a starts with an "z" (pattern matching) 2 [[ $a == "z*" ]] # true if $a is equal to z* 3 4 [ $a == z* ] # file globbing and word splitting take place 5 [ "$a" == "z*" ] # true if $a is equal to z* 6 7 # Thanks, S.C.
- !=
is not equal to
if [ "$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" \> "$b" ]
Note that the ">" needs to be escaped within a [ ] construct.
See Example 26-6 for an application of this comparison operator.
- -z
string is "null", that is, has zero length
- -n
string is not "null".
The -n test absolutely requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7-6) normally works, however, this is an unsafe practice. Always quote a tested string. [1]
Example 7-5. Arithmetic and string comparisons
1 #!/bin/bash 2 3 a=4 4 b=5 5 6 # Here "a" and "b" can be treated either as integers or strings. 7 # There is some blurring between the arithmetic and string comparisons, 8 #+ since Bash variables are not strongly typed. 9 10 # Bash permits integer operations and comparisons on variables 11 #+ whose value consists of all-integer characters. 12 # Caution advised. 13 14 echo 15 16 if [ "$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 [ $string 1 ] has one argument, "]"
49 # if [ "$string 1" ] 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 # Also, thank you, Florian Wisser, for the "heads-up". |
Example 7-7. zmost
1 #!/bin/bash
2
3 #View gzipped files with 'most'
4
5 NOARGS=65
6 NOTFOUND=66
7 NOTGZIP=67
8
9 if [ $# -eq 0 ] # same effect as: if [ -z "$1" ]
10 # $1 can exist, but be empty: zmost "" arg2 arg3
11 then
12 echo "Usage: `basename $0` filename" >&2
13 # Error message to stderr.
14 exit $NOARGS
15 # Returns 65 as exit status of script (error code).
16 fi
17
18 filename=$1
19
20 if [ ! -f "$filename" ] # Quoting $filename allows for possible spaces.
21 then
22 echo "File $filename not found!" >&2
23 # Error message to stderr.
24 exit $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 $NOTGZIP
32 fi
33
34 zcat $1 | most
35
36 # Uses the file viewer 'most' (similar to 'less').
37 # Later versions of 'most' have file decompression capabilities.
38 # May substitute 'more' or 'less', if desired.
39
40
41 exit $? # Script returns exit status of pipe.
42 # Actually "exit $?" unnecessary, as the script will, in any case,
43 # return the exit status of the last command executed. |
- -a
logical and
exp1 -a exp2 returns true if both exp1 and exp2 are true.
- -o
logical or
exp1 -o exp2 returns true if either exp1 or exp2 are true.
These are similar to the Bash comparison operators && and ||, used within double brackets.
1 [[ condition1 && condition2 ]] |
1 if [ "$exp1" -a "$exp2" ] |
Refer to Example 8-3 and Example 26-11 to see compound comparison operators in action.