How the Shell Runs Commands

When you press Enter, Bash does roughly this:

1. Splits the line into words (the command and its arguments).
2. Expands special characters — variables, wildcards, ~ — into their values.
3. Finds the command.
4. Runs it and waits for it to finish.

Where commands come from

A command can be:

• a builtin — part of Bash itself, like cd, echo, and read;
• an external program — a file somewhere on disk, like ls, grep, or git;
• a function or alias you (or your configuration) defined.

Bash finds external programs by searching the folders listed in the PATH variable, in order. type tells you what a command is:

type cd
type echo
type if

cd is a shell builtin
echo is a shell builtin
if is a shell keyword

# illustrative
type ls
echo "$PATH"

ls is /usr/bin/ls
/usr/local/bin:/usr/bin:/bin

Getting help

• man ls — the full manual page for a program (press q to quit, /word to search).
• ls --help — a quick summary for most GNU programs.
• help cd — help for Bash builtins.
• tldr pages (tldr tar) — short, practical examples, if installed.

Exit status

Every command finishes with an exit status: 0 means success, and anything else means failure. $? holds the status of the last command:

ls /this/does/not/exist
echo "exit status: $?"
echo "hello" > /dev/null
echo "exit status: $?"

ls: cannot access '/this/does/not/exist': No such file or directory
exit status: 2
exit status: 0

Exit statuses are how scripts decide what to do next — a whole later lesson covers them.