test command and [ ] syntax in Bash Scripting

Introduction

The test command and [ ] syntax help check conditions in scripts to decide what to do next.

Check if a file exists before reading it.

Compare two numbers to see which is bigger.

See if a variable is empty or not.

Decide what to do based on user input.

Verify if a directory is present before creating files.

Syntax

Bash Scripting

test CONDITION
[ CONDITION ]

Both test and [ ] do the same thing: check a condition.

Make sure to put spaces around [ and ] when using [ ].

Examples

Check if a file named filename.txt exists.

Bash Scripting

test -f filename.txt
if [ -f filename.txt ]; then echo "File exists"; fi

Check if variable var equals 'hello'.

Bash Scripting

if [ "$var" = "hello" ]; then echo "Greeting found"; fi

Check if number num is greater than 10.

Bash Scripting

if test $num -gt 10; then echo "Number $num is greater than 10"; fi

Sample Program

This script checks if a file named myfile.txt exists and prints a message. Then it checks if the number 5 is less than 10 and prints the result.

Bash Scripting

#!/bin/bash

filename="myfile.txt"

if [ -e "$filename" ]; then
  echo "File '$filename' exists."
else
  echo "File '$filename' does not exist."
fi

num=5
if test $num -lt 10; then
  echo "Number $num is less than 10."
else
  echo "Number $num is 10 or more."
fi

Important Notes

Always put spaces after [ and before ] when using [ ] syntax.

Use quotes around variables to avoid errors if they are empty or have spaces.

test and [ ] are interchangeable, but [ ] is more common in scripts.

Summary

test and [ ] check conditions in bash scripts.

Use them to make decisions like if-else.

Remember spaces and quotes to avoid mistakes.