How to Use the Command 'set' (with Examples)

The set command in Unix-like operating systems is a multifaceted tool primarily used in shell scripting and command-line operations. It is designed to configure shell behavior and control the shell environment, allowing users to modify shell options, set positional parameters, and more. The command has several options that cater to different needs, ranging from debugging to script optimization.

Display the Names and Values of Shell Variables

Code:

set

Motivation:

Using the simple set command without arguments is a convenient way to display all shell variables and their current values. This is essential for debugging when you need to inspect your shell environment and ensure that variables are set correctly. It provides a comprehensive listing, making it easier to track the state of the environment.

Explanation:

The command set here has no options or arguments provided, so it outputs all shell variables and functions available in the current shell session. This includes user-defined variables and internal shell variables, offering a snapshot of the environment’s current state.

Example Output:

BASH=/bin/bash
BASH_VERSINFO=([0]="5" ... )
COLUMNS=80
DIRSTACK=()
EUID=1000
GROUPS=()
HOME=/home/user
...

Export Newly Initialized Variables to Child Processes

Code:

set -a

Motivation:

When scripts create variables, they are not automatically exported to child processes. Using set -a is crucial when you want new variables within your script to be inherited by any spawned child process, allowing for seamless communication between your script and its child processes.

Explanation:

The -a option, short for “allexport,” tells the shell to automatically export every variable that’s modified or created. Essentially, it ensures that all variables are available to any child process spawned by the shell during the session.

Example Output:

# After setting
VAR1="hello"
# The variable VAR1 is available to a child process

Write Formatted Messages to stderr When Jobs Finish

Code:

set -b

Motivation:

In a multitasking environment, it can be useful to be informed when background jobs complete. Using set -b enables asynchronous notification of job completions, which helps in keeping track of background processes in long-running scripts or when multitasking in interactive shells.

Explanation:

The -b option stands for “notify” mode. It shifts job termination notifications from when you return to the prompt to when they actually occur, ensuring that you are promptly informed through messages to stderr when jobs finish.

Example Output:

[1]+  Done                    sleep 100 &

Write and Edit Text in the Command-Line with vi-like Keybindings

Code:

set -o vi

Motivation:

Some users prefer the powerful keybindings and editing capabilities of the vi text editor. By using set -o vi, you can bring the familiarity and efficiency of vi commands to the command-line, allowing for quick navigation and editing.

Explanation:

The -o vi option configures the shell to use vi-like keybindings. This mode uses vi’s command and insert modes, enabling features such as using yy to yank (copy) and p to paste.

Example Output:

# After issuing command
# Now in 'vi' keybindings mode for the command-line

Return to Default Mode

Code:

set -o emacs

Motivation:

For users accustomed to the default shell keybindings, which are similar to the minimalistic emacs text editor, set -o emacs restores these controls. It is useful when you need to revert from vi mode or simply prefer the default keybindings for efficiency.

Explanation:

The -o emacs option switches the shell to use emacs-like keybindings. This is the default mode for most shells and allows the use of navigation and editing commands similar to emacs.

Example Output:

# Now in 'emacs' keybindings mode for the command-line

List All Modes

Code:

set -o

Motivation:

When scripting or debugging, it’s helpful to know which shell options are currently active. Using set -o lists all adjustable shell options along with their current status, helping check the current shell configuration.

Explanation:

The command set -o provides a clear listing of all shell options and whether each is turned “on” or “off.” It does not directly alter shell configurations but serves as a diagnostic tool to inform you of the shell’s current settings.

Example Output:

allexport      off
braceexpand    on
emacs          on
...
nounset        off

Exit the Shell When (Some) Commands Fail

Code:

set -e

Motivation:

In scripts where failure of any critical command should halt execution, enabling set -e ensures that your script stops at the first sign of failure. This prevents subsequent commands from running and potentially causing errors or data corruption due to unmet conditions.

Explanation:

The -e option makes the shell exit immediately if any untested command fails. It’s primarily used in script files to catch errors efficiently and prevent the execution of unintended commands following a failure.

Example Output:

# If a command fails
# Script execution halts immediately

Conclusion

The set command is a powerful utility in the Unix shell environment, streamlining many aspects of shell behavior and scripting. Understanding various use cases, from exporting variables to configuring shell options, not only aids in effective scripting but also enhances interactive shell proficiency. By mastering set, users and developers can wield greater control over their shell environments and scripts.