Bash Shell Scripting

Bash If Else

Many times, you need to execute a certain set of commands based on condition and skip others under different conditions. In that situation, use Bash if else statement.

Whenever you run a command, it returns exit status code. This exit status code of the command is used by the if statement to execute a block of statements.

In Bash, zero(0) is considered true and non-zero value is considered false. If you want to check condition in if else then use Bash test command or square brackets([]). There are 4 different forms of if statements in Bash-

Simple if statement
if-then-else statement
else if ladder
Nested if statements

Bash if then statement

if command
then
   commands
fi

Another syntax of Bash if then statement

if command; then
   commands
fi

If the command runs successfully, it will return 0 as the exit status code which is considered true. And all the commands written between then and fi are executed. If the exit status of the command is non-zero, then the commands mentioned between then and fi are skipped.

Bash if-then-else statement

if command
then
   commands
else
   commands
fi

If the command runs successfully and returns an exit status code of zero, then the commands listed between then and else will be executed. And if the command returns a non-zero exit status code, then the commands written in the else section is executed.

Bash else if ladder

if command1
then
   commands
elif command2
then
   commands
elif command3
then
   commands
fi

Bash executes if statements sequentially and any command present in the if statement returns a zero exit status code, a block of commands written just after the immediate then will be executed and all other statements are skipped.

Bash Nested if statement

if command1
then
   commands
else
   if command2
   then
      commands
   else
      commands
   fi
fi

You can write if statement inside else block and if block and thus nested any number of if statement.

Note: Instead of using exit status code of a command Bash provides test command that enables you to evaluate any condition.