test / [ ] / [[ ]]

Use the test command (POSIX-compliant).

test condition

Use [ ] (alternative notation for test).

[ condition ]

Use [[ ]] (Bash extension — recommended).

[[ condition ]]

Expression	Description
-e path	True if the file or directory exists.
-f path	True if a regular file exists.
-d path	True if a directory exists.
-r path	True if the path is readable.
-w path	True if the path is writable.
-x path	True if the path is executable.
-s path	True if the file size is greater than zero.
-L path	True if the path is a symbolic link.
-z "$str"	True if the string length is zero (empty string).
-n "$str"	True if the string length is greater than zero.
"$a" = "$b"	True if strings a and b are equal (in [ ] use =; in [[ ]], == is also accepted).
"$a" != "$b"	True if strings a and b differ.
$a -eq $b	True if integers a and b are equal.
$a -ne $b	True if integers a and b differ.
$a -lt $b	True if a is less than b (less than).
$a -le $b	True if a is less than or equal to b (less or equal).
$a -gt $b	True if a is greater than b (greater than).
$a -ge $b	True if a is greater than or equal to b (greater or equal).
! condition	Negates the condition.
condition1 -a condition2	AND inside [ ] (use && inside [[ ]]).
condition1 -o condition2	OR inside [ ] (use || inside [[ ]]).
[[ $s =~ regex ]]	True if the string matches the regular expression ([[ ]] only).

Use -f (regular file) and -d (directory) to check file attributes.

check_attr.sh

#!/bin/bash
if [ -f "/etc/hosts" ]; then
    echo "/etc/hosts is a regular file"
fi

if [ -d "/tmp" ]; then
    echo "/tmp is a directory"
fi

bash check_attr.sh
/etc/hosts is a regular file
/tmp is a directory

Combine -r (readable) and -s (non-empty) with &&.

check_readable.sh

if [ -r "/etc/hosts" ] && [ -s "/etc/hosts" ]; then
    echo "The file is readable and not empty"
fi

bash check_readable.sh
The file is readable and not empty

Use -z to check for an empty string and -n to check for a non-empty string.

check_string.sh

name="Alice"
if [ -z "$name" ]; then
    echo "Name is not set"
elif [ -n "$name" ]; then
    echo "Name: $name"
fi

bash check_string.sh
Name: Alice

Use operators such as -eq, -gt, and -lt for numeric comparisons.

check_number.sh

count=5
if [ $count -eq 0 ]; then
    echo "Zero"
elif [ $count -gt 0 ] && [ $count -lt 10 ]; then
    echo "Between 1 and 9"
fi

bash check_number.sh
Between 1 and 9

Inside [[ ]], you can use wildcard pattern matching with ==.

check_pattern.sh

filename="report_2026.pdf"
if [[ "$filename" == *.pdf ]]; then
    echo "This is a PDF file"
fi

bash check_pattern.sh
This is a PDF file

Use the =~ operator inside [[ ]] to test against a regular expression.

check_semver.sh

version="v1.2.3"
if [[ "$version" =~ ^v[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
    echo "Semantic version format"
fi

bash check_semver.sh
Semantic version format

You can also use the exit status of the test command directly with &&.

test -e "/tmp" && echo "/tmp exists"
/tmp exists

Spaces inside [ ] are required. Writing [ -f "$file"] without a space before the closing bracket causes an error. Always quote variables with "$var" using double quotes. Without quotes, a value containing spaces is split into multiple arguments, which leads to errors.

Be careful not to mix up the integer comparison operators (-eq -ne -lt -gt -le -ge) with the string comparison operators (= !=). Using -eq with strings produces unexpected results.

For a full overview of if statements, see if / elif / else.