A binary comparison operator compares two variables or quantities. Note that integer and string comparison use a different set of operators.
is equal to
if [ "$a" -eq "$b" ]
is not equal to
if [ "$a" -ne "$b" ]
is greater than
if [ "$a" -gt "$b" ]
is greater than or equal to
if [ "$a" -ge "$b" ]
is less than
if [ "$a" -lt "$b" ]
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.
[[ $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" \> "$b" ]
Note that the “>” needs to be
escaped within a [ ] construct.
See Example 27.11, “The Bubble Sort” for an application of this comparison operator.
string is null, that is, has zero length
String='' # Zero-length ("null") string variable.
if [ -z "$String" ]
then
echo "\$String is null."
else
echo "\$String is NOT null."
fi # $String is null.string is not null.
The -n test
requires that the string be quoted within the
test brackets. Using an unquoted string with
! -z, or even just the
unquoted string alone within test brackets (see Example 7.6, “Testing whether a string is null”) normally works, however, this is
an unsafe practice. Always quote
a tested string.
[37]
Example 7.5. Arithmetic and string comparisons
#!/bin/bash a=4 b=5 # Here "a" and "b" can be treated either as integers or strings. # There is some blurring between the arithmetic and string comparisons, #+ since Bash variables are not strongly typed. # Bash permits integer operations and comparisons on variables #+ whose value consists of all-integer characters. # Caution advised, however. echo if [ "$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 $?" is unnecessary, as the script will, in any case,
#+ return the exit status of the last command executed.
These are similar to the Bash comparison operators && and ||, used within double brackets.
[[ condition1 && condition2 ]]
The -o and -a operators work with the test command or occur within single test brackets.
if [ "$expr1" -a "$expr2" ] then echo "Both expr1 and expr2 are true." else echo "Either expr1 or expr2 is false." fi
But, as rihad points out:
[ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ] # true [ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ] # (no output) # ^^^^^^^ False condition. So far, everything as expected. # However ... [ 1 -eq 2 -a -n "`echo true 1>&2`" ] # true # ^^^^^^^ False condition. So, why "true" output? # Is it because both condition clauses within brackets evaluate? [[ 1 -eq 2 && -n "`echo true 1>&2`" ]] # (no output) # No, that's not it. # Apparently && and || "short-circuit" while -a and -o do not.
Refer to Example 8.3, “Compound Condition Tests Using && and ||”, Example 27.17, “Simulating a two-dimensional array, then tilting it”, and Example A.29, “Spammer Hunt” to see compound comparison operators in action.
[37] As S.C. points out, in a compound test,
even quoting the string variable might not
suffice. [ -n "$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).