Comparison Operators (tricky bash details you might not know) - thanks to Neeraj, thegrendel, and tldp.org

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.

integer comparison

-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"))

string comparison

=

is equal to if [ "$a" = "$b" ]

Note the whitespace framing the =. if [ "$a"="$b" ] is not equivalent to the above.

==

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. [[ $a == z* ]] # True if $a starts with an "z" (pattern matching). [[ $a == "z*" ]] # True if $a is equal to z* (literal matching). [ $a == z* ] # File globbing and word splitting take place. [ "$a" == "z*" ] # True if $a is equal to z* (literal matching). # 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 27-11 for an application of this comparison operator.

-z

string is null, that is, has zero length

String='' # Zero-length ("null") string variable. if [ -z "$String" ] then echo "\$String is null." else echo "\$String is NOT null." 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

#!/bin/bash a=4 b=5 # Here "a" and "b" can be treated either as integers or strings. # There is some blurring between the arithmetic and string comparisons, #+ since Bash variables are not strongly typed. # Bash permits integer operations and comparisons on variables #+ whose value consists of all-integer characters. # Caution advised, however. echo if [ "$a" -ne "$b" ] then echo "$a is not equal to $b" echo "(arithmetic comparison)" fi echo if [ "$a" != "$b" ] then echo "$a is not equal to $b." echo "(string comparison)" # "4" != "5" # ASCII 52 != ASCII 53 fi # In this particular instance, both "-ne" and "!=" work. echo exit 0

Example 7-6. Testing whether a string is null

#!/bin/bash # str-test.sh: Testing null strings and unquoted strings, #+ but not strings and sealing wax, not to mention cabbages and kings . . . # Using if [ ... ] # If a string has not been initialized, it has no defined value. # This state is called "null" (not the same as zero!). if [ -n $string1 ] # string1 has not been declared or initialized. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Wrong result. # Shows $string1 as not null, although it was not initialized. echo # Let's try it again. if [ -n "$string1" ] # This time, $string1 is quoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Quote strings within test brackets! echo if [ $string1 ] # This time, $string1 stands naked. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # This works fine. # The [ ... ] test operator alone detects whether the string is null. # However it is good practice to quote it (if [ "$string1" ]). # # As Stephane Chazelas points out, # if [ $string1 ] has one argument, "]" # if [ "$string1" ] has two arguments, the empty "$string1" and "]" echo string1=initialized if [ $string1 ] # Again, $string1 stands unquoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Again, gives correct result. # Still, it is better to quote it ("$string1"), because . . . string1="a = b" if [ $string1 ] # Again, $string1 stands unquoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Not quoting "$string1" now gives wrong result! exit 0 # Thank you, also, Florian Wisser, for the "heads-up".

Example 7-7. zmore

#!/bin/bash # zmore # View gzipped files with 'more' filter. E_NOARGS=85 E_NOTFOUND=86 E_NOTGZIP=87 if [ $# -eq 0 ] # same effect as: if [ -z "$1" ] # $1 can exist, but be empty: zmore "" arg2 arg3 then echo "Usage: `basename $0` filename" >&2 # Error message to stderr. exit $E_NOARGS # Returns 85 as exit status of script (error code). fi filename=$1 if [ ! -f "$filename" ] # Quoting $filename allows for possible spaces. then echo "File $filename not found!" >&2 # Error message to stderr. exit $E_NOTFOUND fi if [ ${filename##*.} != "gz" ] # Using bracket in variable substitution. then echo "File $1 is not a gzipped file!" exit $E_NOTGZIP fi zcat $1 | more # Uses the 'more' filter. # May substitute 'less' if desired. exit $? # Script returns exit status of pipe. # Actually "exit $?" is unnecessary, as the script will, in any case, #+ return the exit status of the last command executed.

compound comparison

-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 is true.

These are similar to the Bash comparison operators && and ||, used within double brackets.

[[ condition1 && condition2 ]]

The -o and -a operators work with the test command or occur within single test brackets.

if [ "$expr1" -a "$expr2" ] then echo "Both expr1 and expr2 are true." else echo "Either expr1 or expr2 is false." fi

But, as rihad points out: [ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ] # true [ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ] # (no output) # ^^^^^^^ False condition. So far, everything as expected. # However ... [ 1 -eq 2 -a -n "`echo true 1>&2`" ] # true # ^^^^^^^ False condition. So, why "true" output? # Is it because both condition clauses within brackets evaluate? [[ 1 -eq 2 && -n "`echo true 1>&2`" ]] # (no output) # No, that's not it. # Apparently && and || "short-circuit" while -a and -o do not.

Refer to Example 8-3, Example 27-17, and Example A-29 to see compound comparison operators in action.

Notes

[1]

As S.C. points out, in a compound test, even quoting the string variable might not suffice. [ -n "$string" -o "$a" = "$b" ] may cause an error with some versions of Bash if $string is empty. The safe way is to append an extra character to possibly empty variables, [ "x$string" != x -o "x$a" = "x$b" ] (the "x's" cancel out).

Other Comparison Operators

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