⚙️ How to Use the compgen Command in Linux (Step-by-Step Guide)

The compgen command in Linux is a powerful but lesser-known tool used to generate lists of possible command completions. It’s mainly used behind the scenes for tab completion in Bash, but you can also use it directly for scripting and exploration.

🔎 What is compgen?

compgen stands for completion generator.

👉 It outputs lists of:

• Commands
• Aliases
• Built-ins
• Filenames
• Variables

✅ Basic Syntax

compgen [option]

📜 List Available Commands

compgen -c

👉 Shows all executable commands in your $PATH

📁 List Files in Current Directory

compgen -f

📂 List Directories Only

compgen -d

🧠 List Shell Built-ins

compgen -b

🔗 List Aliases

compgen -a

🔣 List Environment Variables

compgen -v

🔍 Filter Results (Search)

compgen -c | grep ssh

👉 Shows commands related to ssh

🎯 Match Specific Prefix

compgen -c ls

👉 Lists commands starting with ls

🚀 Real-World Uses

• Explore available commands quickly
• Build auto-completion scripts
• Debug Bash completion issues
• Generate dynamic lists in scripts

⚠️ Common Notes

• Output can be very large (especially -c)
• Best used with filters like grep
• Primarily useful in Bash scripting

🧠 Pro Tips

• Combine with sort:

  compgen -c | sort

• Count commands:

  compgen -c | wc -l

• Use in scripts for dynamic input suggestions

👍 Quick Summary

• compgen generates completion lists
• Works with commands, files, variables, and more
• Useful for scripting and discovery
• Often used with pipes (|) for filtering

🎯 Final Thoughts

While most users rely on tab completion without thinking about it, compgen gives you direct control over how those completions work. It’s especially useful for advanced users, shell customization, and scripting.