Exploring the 'compgen' Command in Bash (with examples)

The compgen command in Bash is a built-in utility primarily used for autocomplete functionalities. Engineers and developers often press the TAB key twice to trigger auto-completion when they can’t recall a command name or need a quick reminder of available options. The compgen command comes to the rescue by providing lists of applicable suggestions, depending on the arguments provided to it. Through different flags, compgen can list available commands, aliases, functions, and more, facilitating a smoother and more efficient command-line experience. Below, we delve into several use cases that demonstrate the power and versatility of compgen.

Use Case 1: List All Commands That You Could Run

Code:

compgen -c

Motivation: When venturing into unfamiliar systems, especially when one’s usual tools and scripts may not be available, it becomes essential to know all executable commands on the machine. By obtaining a comprehensive list of commands, a user can effectively explore and harness the available resources without stumbling or wasting time on trial and error.

Explanation:

• -c: This option tells compgen to list all the commands that are executable in the current environment. This includes both built-in shell commands and those available in the system’s PATH directories. Listing every available command can guide users to understand the environment’s toolset quickly.

Example Output:

cd
ls
echo
git
awk
python3
...

Use Case 2: List All Aliases

Code:

compgen -a

Motivation: Aliases are shortcuts or abbreviations that map to more extended command sequences, allowing users to execute more complex or frequently used commands with minimal keystrokes. Knowing all the predefined aliases provides insight into shortcuts tailored either by the system administrator or oneself, enhancing command efficiency.

Explanation:

• -a: This flag is used to show all available alias definitions in the current shell session. This can reveal helpful shortcuts that might speed up repetitive tasks or simplify elaborate command sequences.

Example Output:

gs='git status'
ll='ls -alF'
la='ls -A'
...

Use Case 3: List All Functions That You Could Run

Code:

compgen -A function

Motivation: Functions in Bash encapsulate tasks into callable units, enhancing script modularity and reusability. Understanding what functions are ready-to-use can save hours of scripting time. Whether the functions are user-defined or sourced from external scripts, knowing their availability ensures that one can leverage existing solutions efficiently.

Explanation:

• -A function: This argument specifies that compgen should display all shell functions defined within the session. It’s instrumental in environments laden with complex scripts, allowing for a quick cataloging of potentially reusable logic.

Example Output:

my_function
deploy_script
cleanup_temp
...

Use Case 4: Show Shell Reserved Keywords

Code:

compgen -k

Motivation: Shell keywords are the reserved words within the bash language that define its syntax and structure, such as if, then, else, fi, and others. By understanding and listing these keywords, users gain deeper insight into the shell’s functionality and rules they must work within to avoid syntax errors.

Explanation:

• -k: This option instructs compgen to list the shell’s reserved keywords, which are crucial for structuring scripts and understanding which words are internally used by Bash.

Example Output:

if
then
else
do
done
for
while
...

Use Case 5: See All Available Commands/Aliases Starting with ’ls'

Code:

compgen -ac ls

Motivation: Sometimes, one wishes to find commands or aliases based on a specific prefix or pattern; this is helpful when trying to recall the exact command syntax or reviewing available extensions of a command. This approach mitigates the common problem of memory recall, particularly with commands that have numerous options and variations.

Explanation:

• -a: Includes alias matches in the search.
• -c: Includes command matches in the search.
• ls: Acts as the prefix filter; compgen will list only those commands and aliases that start with ls.

Example Output:

ls
lsof
lsblk
lsmod
lsa
lsl
...

Conclusion:

The compgen command is a versatile and powerful tool within the Bash environment, serving as an indispensable aid for users navigating complex command-line ecosystems. Each use case unfolds its utility, from listing basic commands to surfacing elaborate functions and aliases. By mastering compgen, developers and engineers enhance their ability to efficiently interact with operating systems through the terminal, accelerating their workflow and fostering a more intuitive computing experience.