Master Bash Conditional Tests: File, String, and Numeric Checks Explained

Technical advancement is merely quantitative accumulation; intellectual growth is a qualitative leap. — Lao Nian Hai

You can view the test command manual with

man test

.

Test Statement Syntax

The test command can be written in four forms:

test expression

[ expression ]

[[ expression ]]

(( expression ))

Forms 1 and 2 have essentially the same functionality with different syntax. Form 3 supports pattern matching with wildcards, while form 4 is mainly for integer comparisons and arithmetic.

Condition Test Types

File tests

Numeric tests

String tests

Logical tests

File Tests

Common file test operators:

-b : file exists and is a block device

-c : file exists and is a character device

-d : file exists and is a directory

-e : file exists

-f : file exists and is a regular file

-s : file exists and its size is greater than zero

-L : file exists and is a symbolic link

-r : file exists and is readable

-w : file exists and is writable

-x : file exists and is executable

file1 -nt file2 : file1 is newer than file2

file1 -ot file2 : file1 is older than file2

Examples:

<code># Create a regular file
touch wanger
# Check if it is a regular file
test -f wanger && echo 0 || echo 1
# Check if it is a directory
[ -d wanger ] && echo 0 || echo 1
# Create another file
touch wangerxiao
# Compare modification times
test wanger -nt wangerxiao && echo 0 || echo 1
[ wanger -ot wangerxiao ] && echo 0 || echo 1</code>

String Tests

Common string test operators:

-z string : true if the length of

string

is zero

-n string : true if the length of

string

is non‑zero

string1 = string2 : true if the strings are equal

string1 != string2 : true if the strings are not equal

Examples:

<code># Define a variable
name=wanger
# Test if the variable is empty
[ -z $name ] && echo 0 || echo 1
# Test if the variable is non‑empty
test -n $name && echo 0 || echo 1
# Compare variable with a literal
[ $name != "wanger" ] && echo 0 || echo 1
test $name = "wanger" && echo 0 || echo 1</code>

Numeric Tests

Common numeric test operators:

-eq : equal

-ne : not equal

-gt : greater than

-ge : greater than or equal

-lt : less than

-le : less than or equal

Note: These operators cannot be used inside

(( ))

; within

(( ))

you should use

< > = != >= <=

for comparisons.

Examples:

<code># Check if 6 is less than or equal to 5
test 6 -le 5 && echo 0 || echo 1
# Check if 6 is greater than 5
[ 6 -gt 5 ] && echo 0 || echo 1
# Check equality with [[ ]]
[[ 6 = 5 ]] && echo 0 || echo 1
# Arithmetic comparison
((5!=6)) && echo 0 || echo 1</code>

Logical Operators

-a : logical AND (equivalent to

&&

)

-o : logical OR (equivalent to

||

)

! : logical NOT

Examples:

<code># Define a variable
test="123"
# Check if variable is non‑empty OR equals "123"
[ ! -z $test -o $test = "123" ] && echo 0 || echo 1
# Using && and || (only -a and -o are allowed inside [ ])
[ -z $test ] && test $test = "123" && echo 0 || echo 1
# Using [[ ]] with ||
[[ -z $test || $test = "123" ]] && echo 0 || echo 1</code>

For the full original notes, click the original article link or view the learning notes in the menu.