# 7.3. Other Comparison Operators

A binary comparison operator compares two variables or quantities. Note the separation between integer and string comparison.

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" ]

==

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, Stephané 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

-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

```#!/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.

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

# Lets 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 ("\$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 naked.
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 naked.
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'

NOARGS=65
NOTFOUND=66
NOTGZIP=67

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 \$NOARGS
# Returns 65 as exit status of script (error code).
fi

filename=\$1

if [ ! -f "\$filename" ]   # Quoting \$filename allows for possible spaces.
then
# Error message to stderr.
exit \$NOTFOUND
fi

if [ \${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
echo "File \$1 is not a gzipped file!"
exit \$NOTGZIP
fi

zcat \$1 | more

# Uses the filter 'more.'
# 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 are 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 [ "\$exp1" -a "\$exp2" ]`

Refer to Example 8-3 and Example 26-16 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).

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