if command in linux with examples
if command in Linux is used for conditional execution in shell scripts.The if command is essential for writing scripts that perform different actions based on different conditions.
if COMMANDS list is executed, if its status is true, then the then COMMANDS list is executed. Otherwise, each elif COMMANDS list is executed in turn, and if its exit status is true, the corresponding then COMMANDS list is executed, and the if command completes. Otherwise, the else COMMANDS list is executed, if present. The exit status of the entire construct is the exit status of the last command executed, or false if no condition tested true.
Syntax
if: if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fiCommonly used options with if command
Here are some of the most commonly used conditions with the if command:
Condition | Description |
|---|---|
[ -f FILE ] | return TRUE if FILE exists and is a regular file |
[ -d DIR ] | return TRUE if DIR exists and is a directory |
[ -z STRING ] | return TRUE if the length of string is zero(empty string) |
[ -n STRING ] | return TRUE if the length of string is non-zero. |
[ STRING1 = STRING2 ] | return TRUE if STRING1 and STRING2 are identical |
[ NUM1 -eq NUM2 ] | return TRUE if NUM1 is equal to NUM2 |
[ NUM1 -gt NUM2 ] | return TRUE if NUM1 is greater than NUM2 |
Examples of Using the if Command in Shell Scripts
1. -f FILE: Check if a file exists
This example checks if a file named testfile.txt exists.
if [ -f "testfile.txt" ]
then
echo "The file exists."
else
echo "The file does not exist."
fi
2. -d DIR: Checks if a directory exists
This example checks if a directory /home/user exists.
if [ -d "/home/user" ]
then
echo "Directory exists."
else
echo "Directory does not exist."
fi
3. -z STRING: Tests if a string is empty
This example prompts the user to enter a string and checks if it is empty.
#!/bin/bash
echo "Enter a string: "
read string
if [ -z "$string" ]
then
echo "The string is empty."
else
echo "The string is not empty."
fi
4. STRING1=STRING2: Tests if two strings are equal
This example checks if two user-provided strings are identical
echo "Enter string1:"
read string1
echo "Enter string2:"
read string2
if [ "$string1" = "$string2" ]
then
echo "The strings are equal."
else
echo "The strings are not equal."
fi
5. NUM1 -eq NUM2: Tests if two numbers are equal or not
This example checks if two user-provided numbers are equal.
echo "Enter 1st number:"
read num1
echo "Enter 2nd number:"
read num2
if [ "$num1" -eq "$num2" ]
then
echo "The numbers are equal."
else
echo "The numbers are not equal."
fi
6. NUM1 -gt NUM2: Tests if the first number greater than second
This example compares two user-provided numbers.
echo "Enter 1st number:"
read num1
echo "Enter 2nd number:"
read num2
if [ "$num1" -gt "$num2" ]
then
echo "The first number is greater than the second."
else
echo "The first number is not greater than the second."
fi
Example:
Common if Command Option
1. help if:
It displays help information.
help if
