if

if

The if statement in Unix-like operating systems is a fundamental control structure used in shell scripting to perform conditional execution of commands. It allows you to execute a set of commands only if a specified condition is true, and optionally execute another set of commands if the condition is false.

Basic Syntax

if condition
then
  commands
fi

• condition: A command or test expression that returns a true (0) or false (non-zero) exit status.

• commands: The commands to execute if the condition is true.

Extended Syntax

You can extend the if statement with else and elif (else if) clauses to handle multiple conditions:

if condition1
then
  commands1
elif condition2
then
  commands2
else
  commands3
fi

• condition1: The primary condition to test.

• commands1: Commands to execute if condition1 is true.

• condition2: The secondary condition to test if condition1 is false.

• commands2: Commands to execute if condition2 is true.

• commands3: Commands to execute if neither condition1 nor condition2 is true.

Examples

Basic if Statement

if [ $USER == "root" ]
then
  echo "You are the root user."
fi

if-else Statement

if [ $USER == "root" ]
then
  echo "You are the root user."
else
  echo "You are not the root user."
fi

if-elif-else Statement

if [ $USER == "root" ]
then
  echo "You are the root user."
elif [ $USER == "admin" ]
then
  echo "You are the admin user."
else
  echo "You are a regular user."
fi

Practical Use Cases

Checking File Existence

file="/path/to/file"
if [ -f $file ]
then
  echo "$file exists."
else
  echo "$file does not exist."
fi

Checking Directory Existence

directory="/path/to/directory"
if [ -d $directory ]
then
  echo "$directory exists."
else
  echo "$directory does not exist."
fi

Comparing Numbers

num1=10
num2=20
if [ $num1 -lt $num2 ]
then
  echo "$num1 is less than $num2."
else
  echo "$num1 is not less than $num2."
fi

Checking Command Exit Status

ping -c 1 example.com > /dev/null 2>&1
if [ $? -eq 0 ]
then
  echo "example.com is reachable."
else
  echo "example.com is not reachable."
fi

Advanced Examples

Nested if Statements

if [ $USER == "root" ]
then
  if [ -f "/etc/passwd" ]
  then
    echo "Root user and /etc/passwd exists."
  else
    echo "Root user but /etc/passwd does not exist."
  fi
else
  echo "You are not the root user."
fi

Combining Conditions with && and ||

if [ $USER == "root" ] && [ -f "/etc/passwd" ]
then
  echo "Root user and /etc/passwd exists."
else
  echo "Either not root user or /etc/passwd does not exist."
fi

Using test Command

The test command (or its alias [ ... ]) is used to evaluate expressions:

if test $USER = "root"
then
  echo "You are the root user."
fi

Conclusion

The if statement is a crucial tool in shell scripting, enabling conditional execution of commands based on the evaluation of expressions. By using if, else, and elif clauses, you can create scripts that respond dynamically to different conditions and states.

help

-n: This option tests if a variable is not empty.
-z: This option tests if a variable is empty.
-e: This option tests if a file or directory exists.
-f: This option tests if a file is a regular file.
-d: This option tests if a file is a directory.