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

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

Using if/then with Any Command

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

Basic if/then Syntax

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

Extended if/then with elif

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

Arithmetic Tests with (( ))

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

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

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

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

#!/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