RHCSA - Create Simple Shell Scripts: Conditionally Execute Code (use of: if, test, [], etc.)

In Linux, you can conditionally execute code using commands such as if, test, and []. Conditional execution allows you to run specific code blocks based on certain conditions.

Using Conditions with Tests

The if statement is used for conditional branching in Linux shell scripting. Here's the basic syntax of an if statement:

if [ condition ]
then
    # code to execute if the condition is true
else
    # code to execute if the condition is false
fi

In the above syntax, the condition is an expression that evaluates to either true or false. If the condition is true, the code block inside the then section will be executed. Otherwise, if the condition is false, the code block inside the else section will be executed.

The else section is not required so if you do not need to have an action take place when the if statement resolves to false, then do not add an else block.

You can use the test command or square brackets [] to evaluate conditions.

Check if the /etc/passwd file exists using both test and [] methods

if test -f /etc/passwd
then
    echo "file.txt exists and is a regular file"
fi

if [ -f /etc/passwd ]
then
    echo "file.txt exists and is a regular file"
fi

The -f option is used to check if file.txt exists and is a regular file. If the condition is `true``, the message "/etc/passwd exists and is a regular file" will be displayed.

You can also combine conditions using logical operators such as -a (logical AND) and -o (logical OR) within the test command or [].

Check if the var variable is greater than 10 AND less than 20:

var=15

if [ "$var" -gt 10 -a "$var" -lt 20 ]
then
    echo "The value of var is between 10 and 20."
else
    echo "The value of var is NOT between 10 and 20."
fi

Change the value of var to fully test the condition.

Check if the var variable is less than 10 OR greater than 20:

var=8

if [ "$var" -lt 10 -o "$var" -gt 20 ]
then
    echo "The value of var is NOT between 10 and 20."
else
    echo "The value of var is between 10 and 20."
fi

Change the value of var to fully test the condition.

Exit Codes

If a command returns with a zero exit status, the command succeeded. If the command returns a non-zero exit code, the command failed.

Some commands may still have succeeded with a non-zero exit code. Always consult the man pages if unsure.

To check the exit code, run echo $? immediately after the command.

Checking the success of the command grep by checking for bash in /etc/passwd:

grep bash /etc/passwd

echo $?

The output of the echo command should return 0 as bash was found in the /etc/passwd file.

Checking the failure of the command grep by checking for trash in /etc/passwd:

grep trash /etc/passwd

echo $?

The output of the echo command should return 1 as trash was NOT found in the /etc/passwd file.

Using Conditions with Commands

In addition to using conditions with tests, you can also use conditions with commands.

if command
then
    # code to execute if the command succeeds (returns a zero exit status)
else
    # code to execute if the command fails (returns a non-zero exit status)
fi

Attempt to update the timestamp on the /etc/passwd file:

if touch /etc/passwd 2>/dev/null
then
    echo "Updated timestamp"
else
    echo "Failed to update timestamp"
fi

If performed as a normal user, the command will fail, printing to screen "Failed to update timestamp". Switch to the root user with sudo -i and try running again to receive the message "Updated timestamp". Type exit when done to return to your non-root account.

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