Other Comparison Operators

Advanced Bash-Scripting Guide:
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# 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).

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