Day29-Command-Types.md

Day 29 of Cisco Linux Essentials Course

Today I covered Section 5.5: Command Types — learning how Bash decides what to execute when a command is entered.

Internal vs External Commands

Internal (Built-ins) like cd and pwd are loaded directly into RAM along with the shell process. They execute instantly and never need to search $PATH.

External commands like ls, grep, and python3 are executable files stored in directories like /usr/bin. Bash uses $PATH to search for them, directory by directory.

Diagnostic Tools: type vs which

• which — only searches $PATH for the binary file
• type — tells you the exact nature of a command: Built-in, External, Alias, or Function
• type -a — shows every existing version of a command at once (e.g., echo exists both as a shell builtin and as a /bin/echo binary)

Aliases vs Functions

• Aliases (alias c='clear') — single-line shortcuts
• Functions (sys_cleanup () { ... }) — mini-scripts that run multiple complex commands sequentially with a single call

Persistence note: Custom aliases and functions created in a session are temporary. To make them permanent across reboots, add them to shell initialization files like .bashrc.

Tested hands-on on my AWS EC2 instance.

#Linux #DevOps #CiscoNetworkingAcademy #LearningInPublic #Bash