Shell anatomy — commands, options, arguments, exit codes · Linux

Every shell command line follows the same grammar: a command, options, arguments. Producing results is only half the job; reporting whether those results succeeded — via exit codes and stderr — is the other half. Production scripts are built on both halves.

Anatomy of a command line

$ command -o --long-option=value positional-arg another-arg

The parts:

Part	Example	Purpose
Command	`ls`	The program to run. Resolved via `PATH` (or a builtin).
Short option	`-l`	Single-letter flag. Usually a single hyphen.
Long option	`--long`	Full-word flag. GNU-style double hyphen.
Option argument	`--target=eth0`	Some options take a value, attached with `=` or space.
Positional argument	`/var/log`	Required input to the command.
Compound command	`cmd1	cmd2`

Read-only / Safehow is this resolved?— Type tells you whether a command is an external program (resolved via PATH) or a shell builtin (executed by the shell itself). cd has to be a builtin because a child process cannot change the parent shell's working directory.

$ type ls; type cd; type sudo

ls is /usr/bin/ls
cd is a shell builtin
sudo is /usr/bin/sudo

Exit codes

After every command finishes, the shell records an exit code — a small integer. The convention is universal:

0 — success. The command did what you asked.
1 to 125 — failure. The exact value is command-specific but follows conventions: 1 is generic failure, 2 is misuse (often bad arguments), 126 is “command found but not executable”, 127 is “command not found”.
128+N — the command was killed by signal N. 130 means Ctrl-C (SIGINT, signal 2); 137 means SIGKILL (signal 9); 143 means SIGTERM (signal 15).

Read-only / Safeexit codes— true exits 0; false exits 1. $? holds the last command's exit code.

$ true; echo $?; false; echo $?

0
1

Production scripts depend on exit codes. The conditional constructs &&, ||, and if all read $?.

Read-only / Safeconditional chain— grep -q returns 0 if the pattern is found. && runs the next command only on success. || runs the next command only on failure. Chain short-circuits: if grep fails, echo 'missing' runs.

$ grep -q 'root' /etc/passwd && echo found || echo missing

found

Three output channels

A command can produce output on three separate channels:

Channel	Default destination	Default behaviour	Convention
stdout (file descriptor 1)	terminal	Display normally	Program’s normal output
stderr (file descriptor 2)	terminal	Display normally	Diagnostics, warnings, errors
exit code	shell variable `$?`	Invisible	Success / failure as an integer

A robust script redirects stderr separately from stdout because pipelines should not mix normal output with errors:

Read-only / Safesilence permission errors— find prints permission denied on stderr. 2>/dev/null discards them so wc -l only counts successful matches. Important pattern for scripts that touch files they don't own.

$ find /etc -name '*.conf' 2>/dev/null | wc -l

Illustrative output

Command resolution

When you type ls, the shell has to find a program to run. The order:

Shell functions (defined with function name { … }).
Builtins (cd, echo, type, read, printf, test, [, [[).
Lookups in $PATH, left to right.

Anatomy of a command line

Exit codes

Three output channels

Command resolution

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