Command Types

  • Commands in the shell come from different sources.
  • Use the type command to check where a command comes from. Example type command_name ; tells whether it is internal, external, alias, or function

Command Sources

1. Internal Commands

Key Points

  • Internal commands = built into the Bash shell.
  • do NOT require starting a separate program.
  • Shell already knows how to run them.
  • Bash executes it directly.
  • use type to check a check is internal.
  • Example: cd, echo, pwd, history

Example: type cd Output: cd is a shell builtin

2. External Commands

  • Stored outside the shell (not built-in).
  • Shell searches $PATH to find them.
  • Can run them using full path.

Useful Commands & Examples

Purpose Command Output / Meaning
Find full path of a command which ls /bin/ls
Check full path for another command which cal /usr/bin/cal
Run command using full path /bin/ls Executes ls directly
Show how shell interprets the command type cal cal is /usr/bin/cal
Difference (builtin vs external) type echo echo is a shell builtin
which echo /bin/echo
Show all locations of a command type -a echo Shows builtin + external path

3. Aliases

  • Map a longer commands to shorter sequences.
  • Shell substitutes alias with full command before execution.
  • Each shell has its own aliases; new shells don’t inherit them.
  • View current aliases: alias
  • Create new alias: alias name="command"
  • Identify aliases : type

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4. Functions

  • Combine multiple commands into a single callable unit.
  • Can create new commands or override existing ones.
  • Usually defined in shell scripts or initialization files.
  • Execute the function by typing its name.

Syntax:

function_name () {
    commands
}

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  • Internal commands – Built into the shell
  • External commands – Stored as files in system directories
  • Aliases – Custom shortcuts for commands
  • Functions – User-defined command blocks inside the shell