Conditional Expressions

if-then statement

Description

The if statement allow you to test whether a condition is true or false in your script. If the test evaluates to true, then the script will execute any command that is enclosed within the if statement block. If it is false, the commands inside that block will be ignored.

Format

if TEST-COMMAND; 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. Unary expressions are often used to examine the status of a file. If the FILE argument to one of the primaries is of the form /dev/fd/N, then file descriptor “N” is checked. stdin, stdout and stderr and their respective file descriptors may also be used for tests.

The CONSEQUENT-COMMANDS list that follows the then statement can be any valid UNIX command, any executable program, any executable shell script or any shell statement, with the exception of the closing fi. It is important to remember that the then and fi are considered to be separated statements in the shell. Therefore, when issued on the command line, they are separated by a semi-colon.

Examples

#!/bin/bash
read value

# The use of double-brackets only works in BASH, KSH, and ZSH shells
if [[ $value < 5 ]]; then
    echo "Less than 5."
fi

# The use of single-brackets requires a system that is POSIX compliant
if [ "$value" -gt 5 ]; then
    echo "Greater than 5."
fi

# The test built-in accepts several switches which will be described later
if test "$value" -eq 5; then
    echo "Equal to 5."
fi

if-then-else statement

Description

This statement allows you to define an alternate action if the condition being evaluated is false.

Format

if TEST-COMMAND; then
  CONSEQUENT-COMMANDS;
else
  ALTERNATE-CONSEQUENT-COMMANDS;
fi

Like the CONSEQUENT-COMMANDS list following the then statement, the ALTERNATE-CONSEQUENT-COMMANDS list following the else statement can hold any UNIX-style command that returns an exit status.

Examples

#!/bin/bash
if [ $(whoami) ] != 'root' ] && [ -f "$1" ]; then
    echo "You entered $0 $1"
else
    echo "Root can'd do that!" # tongue in cheek
fi

if-then-elif-else statement

Description

This construct allows you to test for a secondary condition if the first condition evaluated to false but before the catch-all else clause.

Format

if TEST-COMMAND; then
  CONSEQUENT-COMMANDS;
elif TEST-COMMAND; then
  MORE-CONSEQUENT-COMMANDS;
else
  ALTERNATE-CONSEQUENT-COMMANDS;
fi

The TEST-COMMANDS list is executed, and if its return status is zero, the CONSEQUENT-COMMANDS list is executed. If TEST-COMMANDS returns a non-zero status, each elif list is executed in turn, and if its exit status is zero, the corresponding MORE-CONSEQUENT-COMMANDS is executed and the command completes. If else is followed by an ALTERNATE-CONSEQUENT-COMMANDS list, and the final command in the final if or elif clause has a non-zero exit status, then ALTERNATE-CONSEQUENT-COMMANDS is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.

Examples

#!/bin/bash
if [ $(whoami) ] != 'root' ] && [ -f "$1" ]; then
    echo "You entered $0 $1, and $1 is a file."
elif [ $(whoami) ] != 'root' ] && [ -d "$1" ]; then
    echo "Error: You entered $0 $1, and $1 is a directory."
else
    echo "Either you are running as root, or your argument was not a file or
    directory."
fi

case statement

Description

Nested if statements might be nice, but as soon as you are confronted with a couple of different possible actions to take, they tend to get confusing.

Each case is an expression matching a pattern. The commands in the COMMAND-LIST for the first match are executed. The “|” symbol is used for separating multiple patterns, and the “)” operator terminates a pattern list. Each case plus its according commands are called a clause. Each clause must be terminated with “;;”. Each case statement is ended with the esac statement.

Format

case EXPRESSION in
1)
COMMAND-LIST
;;
2)
COMMAND-LIST
;;
.
.
.
N)
COMMAND-LIST
;;
*)
COMMAND-LIST
;;
esac

Example

#!/bin/bash

echo 'Options:'
echo '1) ping 127.0.0.1'
echo '2) ping google'
echo '3) ping yahoo'
echo
echo 'Select an option from the menu'
read OPTION

case $OPTION in
    1)
        ping -c 1 "127.0.0.1"
        ;;
    2)
        ping -c 1 "www.google.com"
        ;;
    3)
        ping -c 1 "www.yahoo.com"
        ;;
    *)
        echo "The option selected was not in the list."
        ;;
esac

Primary Expressions

Now that you’ve read about the basics of conditional expressions, the following table provides some switches you can include in the TEST-COMMAND between the if/then or elif/then keywords.

Boolean Operators

Using boolean operators, we can often shorten or circumvent the need for nested if statements. The example on this slide will only evaluate the 2nd expression if the 1st expression evaluates to true.

&& (AND)

Statements following this will only evaluate if the preceding expression evaluates to true

Formats:

EXPR1 && EXPR2

Example:

#!/bin/bash
echo "First Line" && echo "Second Line"

#!/bin/bash
if [ $(whoami) != 'root' ]; then
    if [ -f $1 ]; then
        echo "You entered $0 $1"
    fi
fi

can now be shortened to…

#!/bin/bash
if [ $(whoami) != 'root' ] && [ -f $1 ]; then
    echo "You entered $0 $1"
fi

|| (OR)

Statements following this will only evaluate if the preceding expression evaluates to false

Format:

EXPR1 || EXPR2

Example

#!/bin/bash
if [ $(whoami) != 'user1' ]; then
    echo "User is authorized."
fi
if [ $(whoami) == 'user2' ]; then
    echo "User is authorized."
fi

can now be shortened to…

#!/bin/bash
if [ $(whoami) == 'user1' ] || [ $(whoami) == 'user2' ]; then
    echo "User is authorized."
fi

! (NOT)

An expression preceded by this character will be true if the expression itself evaluates to false.

Format

! EXPR

Example:

if [[ ! $(whoami) == 'root' ]]; then
    echo "User is not root."
fi