Unix Shell scripting conditional expressions

In this post, lets discuss the use of conditionals in bash scripts.

What are conditionals?

At times you need to specify different courses of action to be taken in a shell script, depending on the success or failure of a command. The if construction allows you to specify such conditions.

The most compact syntax of the if command is:

if TEST-COMMANDS; then CONSEQUENT-COMMANDS; fi

The TEST-COMMAND list is executed, and if its return status is zero, the CONSEQUENT-COMMANDS list is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.

The TEST-COMMAND often involves numerical or string comparison tests, but it can also be any command that returns a status of zero when it succeeds and some other status when it fails.

Syntax:

if [ expression 1 ];
then
  Statement(s) to be executed if expression 1 is true
elif [ expression 2 ];
then
   Statement(s) to be executed if expression 2 is true
elif [ expression 3 ];
then
   Statement(s) to be executed if expression 3 is true
else
   Statement(s) to be executed if no expression is true
fi

Example:

#!/bin/sh
a=1
b=2

if [ $a == $b ];
then
   echo "a is equal to b"
elif [ $a -gt $b ];
then
   echo "a is greater than b"
elif [ $a -lt $b ];
then
   echo "a is less than b"
else
   echo "None of the conditions matched"
fi

Now let us see the different ways to use expressions with numeric and strings.

       ( EXPRESSION )

              EXPRESSION is true

       ! EXPRESSION

              EXPRESSION is false

       EXPRESSION1 -a EXPRESSION2

              both EXPRESSION1 and EXPRESSION2 are true

       EXPRESSION1 -o EXPRESSION2

              either EXPRESSION1 or EXPRESSION2 is true

       -n STRING

              the length of STRING is nonzero

       STRING equivalent to -n STRING

       -z STRING

              the length of STRING is zero

       STRING1 = STRING2

              the strings are equal

       STRING1 != STRING2

              the strings are not equal

       INTEGER1 -eq INTEGER2

              INTEGER1 is equal to INTEGER2

       INTEGER1 -ge INTEGER2

              INTEGER1 is greater than or equal to INTEGER2

       INTEGER1 -gt INTEGER2

              INTEGER1 is greater than INTEGER2

       INTEGER1 -le INTEGER2

              INTEGER1 is less than or equal to INTEGER2

       INTEGER1 -lt INTEGER2

              INTEGER1 is less than INTEGER2

       INTEGER1 -ne INTEGER2

              INTEGER1 is not equal to INTEGER2

Expressions for file operations can be found in this link: File Operations

All these details could be obtained from 'man test' command on linux/unix terminal.