Master Bash test, [, and [[: Uncover Truthy and Falsy Logic in Shell Scripts

Understanding Bash test

The

test

command in Bash often confuses beginners, but once grasped it reveals no deep mystery.

Real‑world example

<code>export NVM_DIR="/Users/jero/.nvm"
[ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"  # This loads nvm
</code>

In the snippet above, the brackets

[ ... ]

perform a test that checks whether the

nvm.sh

script exists before sourcing it.

<code>if [ "$(uname)" == 'Darwin' ]; then
  OS='Mac'
elif [ "$(expr substr $(uname -s) 1 5)" == 'Linux' ]; then
  OS='Linux'
else
  echo "Your platform ($(uname -a)) is not supported."
  exit 1
fi
</code>

This

if

statement also relies on

[ ... ]

as a test to determine the operating system.

test and [

Both

test

and

[

evaluate an expression using

expr

and return 0 for true or 1 for false. They are interchangeable:

<code>test 3 -gt 4 && echo True || echo False  # False
[ 3 -gt 4 ] && echo True || echo False  # False
[ 'abc' != 'def' ]; echo $?               # 0
</code>

The special variable

$?

holds the exit status of the previous command. The operator

-gt

means “greater than”.

Numeric comparison operators

-gt

: greater than

-ge

: greater than or equal

-eq

: equal

-ne

: not equal

-lt

: less than

-le

: less than or equal

Examples of return codes

<code>[ 3 -gt 4 ]; echo $?   # 1 (false)
[ 3 > 4 ]; echo $?     # 0 (true, string comparison)
[[ 3 > 4 ]]; echo $?   # 1 (false, arithmetic comparison)
</code>

The double‑bracket

[[

construct behaves more like C‑style expressions and avoids some pitfalls of

[

.

Using [[ ]]

[[

is a keyword, not an external command, and its syntax is closer to C‑family languages. It supports string pattern matching and unary operators:

<code>[[ (-d "$HOME") && (-w "$HOME") ]] && echo 'home is a writable directory'
[[ "abc def .d,x--" == a[abc]*\ ?d* ]]; echo $?   # 0
[[ "abc def c" == a[abc]*\ ?d* ]]; echo $?      # 1
</code>

Common unary operators

-z

: test for empty string

-n

: test for non‑empty string

-d

: directory exists

-e

: file exists

-f

: regular file

-r

: readable

-w

: writable

-N

: modified since last read

Arithmetic evaluation with (( ))

The double‑parenthesis construct is primarily for arithmetic evaluation. Its exit status follows the convention that a result of zero is considered false (exit code 1) and any non‑zero result is true (exit code 0).

<code>(( 0 )); echo $?   # 0 (true)
(( 3 > 4 )); echo $? # 0 (true because expression evaluates to 0)
(( 0 && 1 )); echo $? # 0
(( 0 || 1 )); echo $? # 1
(( 100 && 11 )); echo $? # 1
</code>

References

http://www.ibm.com/developerworks/cn/linux/l-bash-test.html

http://tldp.org/LDP/abs/html/testconstructs.html

http://tldp.org/LDP/abs/html/dblparens.html