How to use the command exec (with examples)

The exec command is used to replace the current process with another process. It allows you to execute a new command while maintaining the same process ID (PID), which can be useful in various scenarios.

Use case 1: Replace with the specified command using the current environment variables

Code:

exec command -with -flags

Motivation:

• This use case is helpful when you want to replace the current process with another command while retaining the current environment variables. By executing the new command in the same environment, you can ensure that it has access to all the relevant settings, configurations, and variables.

Explanation:

• exec is the command itself.
• command -with -flags represents the desired command you want to replace the current process with, along with any necessary flags or options.

Example output: Assuming the current process is a shell running with the environment variable HOME=/user/me, and we execute the following command using exec:

exec echo $HOME

The output will be /user/me, indicating that the new process (echo) inherited the HOME environment variable from the previous process.

Use case 2: Replace with the specified command, clearing environment variables

Code:

exec -c command -with -flags

Motivation:

• In certain cases, you may want to create a fresh environment for the new command, without any inherited environment variables. This use case allows you to clear the existing environment and run the specified command in a clean slate.

Explanation:

• -c is an option flag that instructs exec to clear the environment before executing the command.
• command -with -flags represents the desired command you want to replace the current process with, along with any necessary flags or options.

Example output: If the current process has environment variables HOME=/user/me and LANG=en_US.UTF-8, and we execute the following command using exec:

exec -c echo $HOME

The output will be an empty line since the exec cleared the environment before executing the echo command.

Use case 3: Replace with the specified command and login using the default shell

Code:

exec -l command -with -flags

Motivation:

• This use case is particularly useful when you want to switch to another command while maintaining a login shell. By using the -l option, the new command will be executed using the default shell, offering the same login experience as the current process.

Explanation:

• -l is an option flag that tells exec to use the default shell as the login shell for the new command.
• command -with -flags represents the desired command you want to replace the current process with, along with any necessary flags or options.

Example output: Let’s assume the current process is a login shell running with the default shell set to bash. If we execute the following command using exec:

exec -l echo $SHELL

The output will be /bin/bash, indicating that the new process inherits the default shell setting.

Use case 4: Replace with the specified command and change the process name

Code:

exec -a process_name command -with -flags

Motivation:

• Changing the process name can be valuable in various scenarios, such as when you want to differentiate multiple instances of the same command or give a descriptive name to identify the process in system monitoring tools.

Explanation:

• -a is an option flag that allows you to specify a custom process name for the new command.
• process_name represents the desired name you want to assign to the new process.
• command -with -flags represents the desired command you want to replace the current process with, along with any necessary flags or options.

Example output: If we execute the following command using exec:

exec -a custom_name sleep 5

The new process will be named custom_name, and it will simply sleep for 5 seconds before terminating. This can be verified by checking the process list using tools like ps or system monitoring software.