Master Bash Test Commands: Conditional Expressions, Comparisons, and File Checks

Shell Test Command Overview

Every programming language can evaluate a condition and act based on the result; Bash provides the

test

command, various bracket operators, and the

if/then

construct for this purpose.

Related Commands and Symbols

If/then conditional statements.

Built‑in

[]

and enhanced

[[]]

brackets.

Arithmetic evaluation with

(( ))

and

let

.

Checking Built‑in Types

<code>type test   # test is a shell builtin</code>
<code>type [      # [ is a shell builtin</code>
<code>type [[     # [[ is a shell keyword</code>
<code>type let    # let is a shell builtin</code>

Using if/then with Any Command

<code>#!/bin/bash
if cmp a.txt b.txt &gt; /dev/null
then
    echo 'Files a and b are identical.'
else
    echo 'Files a and b are diff.'
fi</code>

Basic if/then Syntax

<code>if [ condition-true ]
then
    command1
    command2
    ...
else
    # optional elif ...
    command3
    command4
    ...
fi</code>

Extended if/then with elif

<code>if [ condition1 ]
then
    command1
    command2
    command3
elif [ condition2 ]
then
    command4
    command5
else
    default-command
fi</code>

Arithmetic Tests with (( ))

<code>#!/bin/bash
var1=7
var2=10
if (( var1 &gt; var2 ))
then
    echo "var1 is greater than var2"
else
    echo "var1 is less than var2"
fi
exit 0</code>

Numeric Comparison Operators

Common test operators for numbers include:

-eq

: equal

-ne

: not equal

-gt

: greater than

-ge

: greater than or equal

-lt

: less than

-le

: less than or equal

<code>#!/bin/bash
num1=6
num2=9
if [ $num1 -ne $num2 ]
then
    echo "$num1 is not equal to $num2"
fi</code>

String Comparison Operators

str1 = str2

: strings are identical

str1 != str2

: strings differ

-z str1

: string is empty

-n str1

: string is non‑empty

str1 > str2

: string1 sorts after string2

str1 < str2

: string1 sorts before string2

File Attribute Tests

-e file

: file exists

-r file

: file is readable

-w file

: file is writable

-x file

: file is executable

-s file

: file is non‑empty

-d file

: file is a directory

-f file

: file is a regular file

-c file

: file is a character device

-b file

: file is a block device

<code>#!/bin/bash
device1="/dev/sda1"
if [ -b "$device1" ]
then
    echo "$device1 is a block device."
fi

device2='/dev/tty0'
if [ -c "$device2" ]
then
    echo "$device2 is a character device."
fi</code>