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The test Command

To perform tests, such as comparisons, bash provides the test command.

Test accepts parameters that comprise a test, such as a test for string equality, and returns a status code of 0 if the test succeeds or non-0 if it fails:

test "$NAME" == "Chris"

This is used with the if statement like this:

if test "$NAME" == "Chris"
then
  SUPERPOWERS="Yes"
fi

However, this syntax is a bit ugly! So bash provides a synonym for test which is [[ ]]:

if [[ "$NAME" == "Chris" ]]
then
  SUPERPOWERS="Yes"
fi

Remember that the double square-brackets are really just the test command in disguise. This means that the arguments are command arguments, and need to be separated by spaces, just as they would with any other command such as ls or cp.

Note: The original test command in the original Unix shell (the Bourne shell) was aliased to the single square bracket [ ] For compatibility, this is still available in the bash shell, and it has the advantage of being a Posix standard which is consitent across various Posix-compliant shells. However, the double square-bracket version of the test command has some improved capabilities, and is recommended (except when you need a script that is compatible with a wide range of different shells).

Available Tests​

There are four main types of tests available:

Tests Group 1: Filesystem Entries​

These tests check a filename to see if it is a regular file, a directory, or a symbolic link:

[[ -f filename ]]   # True if filename is a regular file
[[ -d filename ]]   # True if filename is a directory
[[ -L filename ]]   # True if filename is a symbolic link

Notes:

  • The -f and -d tests are lowercase, but the -L test is UPPERCASE.

  • If a symbolic link points to a file, then both the -f and -L tests will succeed. If a symbolic link points to a directory, then both the -d and -L tests will succeed.

Tests Group 2: File Permissions​

These tests check a filename to see if the person running the script can read, write, or execute the file (or access the directory):

[[ -r filename ]]   # True if filename is readable
[[ -w filename ]]   # True if filename is writable
[[ -x filename ]]   # True if filename is executable

Tests Group 3: Strings​

These tests accept two string arguments, which are compared:

[[ string1 == string2 ]]   # True if the strings are equal
[[ string1 != string2 ]]   # True if the strings are not equal
[[ string1 > string2 ]]    # True if string1 sorts after string2 lexicographically
[[ string1 < string2 ]]    # True if string1 sorts before string2 lexicographically

A note on the term lexicographically: Sorting lexicographically means sorting according to character code. This is like sorting alphabetically, but it applies to non-alphabetic characters as well, such as digits and punctuation marks. See the manpage for "ascii" to see the sequence of the first 128 character codes (or refer to a Unicode table for all of the character codes).

When sorting lexicographically, a comes before aa, which comes before b:

a
aa
b

In a similar way, 1 comes before 11, which comes before 2:

1
11
2

Note that this is different from numeric order, where 2 would preceed 11:

1
2
11

Test Group 4: Integers​

These tests accept to integer arguments, which are compared:

[[ integer1 -eq integer2 ]]  # Integers are equal
[[ integer1 -ne integer2 ]]  # Integers are not equal
[[ integer1 -gt integer2 ]]  # Integer1 is greater than integer2
[[ integer1 -ge integer2 ]]  # Integer1 is greater than or equal to integer2
[[ integer1 -lt integer2 ]]  # Integer1 is less than integer2
[[ integer1 -le integer2 ]]  # Integer1 is less than or equal to integer2

Other Tests​

The four groups of tests above will cover the vast majority of situations. There are additional tests available to test other conditions, such as whether a variable is defined, or a file refers to a device. See the man page for bash(1) for more information if you're interested in other tests: man bash

Negating and Combining Tests​

The ! operator can be used to negate a test:

[[ ! -f "$FILE" ]]   # True if "$FILE" is not a file (doesn't exist, or is a directory)

The AND operator && combines tests, with the result being True if the tests on the left and right are both true:

[[ $A -lt $B && $B -lt $C ]]  # True if A is less than B, and also B is less than C.

The OR operator || combines tests, with the result being True if either of the tests are true:

[[ $A -lt 0 ]]  # True if either: A is greater than B, or if A is less than 0.

You can use multiple !, &&, and || operators toegether:

[[ -f "$FILE" && -r "$FILE" && -w "$FILE" ]] # True if "$FILE" is a readable, writable regular file.

Tips on Using Tests​

  • Remember to quote any arguments which include whitespace, or which may be null (empty).
  • Be careful with the < and > comparison operators: if you have a syntax error, you may accidentally redirect data (which in the case of > might overwrite a file!)