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

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

In the snippet above, the brackets [ ... ] perform a test that checks whether the nvm.sh script exists before sourcing it.

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

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:

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

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

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

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:

[[ (-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

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).

(( 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

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