Finding Help, Your Environment, and Command Locations

Unix systems are easier to use when you can answer three questions yourself: what does this command do, what will the shell run, and what settings is the program receiving? The core ideas are shared across POSIX-like systems, while manuals, shells, startup files, and command options vary among Linux distributions, BSDs, and macOS.

Read the Local Documentation

man displays installed manual pages. A page name can appear in more than one section, so the section identifies its kind.

Section	Typical contents
1	User commands
2	System calls
3	Library functions
5	File formats and configuration files
8	System administration commands

man ls             # read the ls manual
man 5 passwd       # the passwd file format, not the passwd command
man -k permission  # search manual-page descriptions (often called apropos)
apropos permission # equivalent search where available
whatis ls          # one-line description for an exact name

Press q to leave the usual manual-page viewer; /word searches in it. Minimal installations may not include all pages, and apropos can require a locally built index. man -k and whatis are related but not identical searches, so read their local manuals when results surprise you.

Many programs offer --help, but that long option is a widespread convention, not a POSIX guarantee. Some BSD and macOS utilities differ from GNU/Linux versions in options and output. Prefer man command for the system you are actually using. GNU systems may also have info coreutils 'ls invocation'; the info reader and its manuals are not installed everywhere.

Commands Are Not Always Files

The shell can run a built-in command, a keyword, a function, an alias, or an executable file. Consequently, which can be misleading: on some systems it is an external program and may only search PATH, missing an alias, function, or built-in. Use shell-aware inspection first.

# POSIX shell: print how the shell would resolve a name
command -V cd
command -V printf
command -V ls

# Common Bash/Zsh built-in; details vary by shell
type cd
type -a ls

# Bash has built-in help for its built-ins
help cd

command -v name is commonly used in portable shell scripts to test whether a command is available, but its exact output is shell-dependent. It is preferable to parsing which. Shell aliases are often interactive conveniences, and functions can shadow a command; scripts should not assume either exists.

PATH and Executable Lookup

When you type a simple command name without a slash, the shell searches the colon-separated directories in PATH, generally from left to right. A command containing a slash, such as ./tool or /usr/bin/env, names a path directly instead.

printf '%s\n' "$PATH"
command -v sh
command -V sh

# Show each PATH directory on its own line
printf '%s\n' "$PATH" | tr ':' '\n'

The first matching executable normally wins. Avoid adding the current directory (.) at the beginning of PATH: a program placed in the current directory could unexpectedly run instead of a trusted system command. Use ./program when you intentionally mean the local file.

Environment Variables

An environment variable is a name/value setting inherited by child processes. A shell variable is not inherited until exported. Program-specific meanings vary, so setting a variable does not guarantee every program will use it.

project='training'
printf '%s\n' "$project"       # shell variable
export project                  # children now receive it
env | grep '^project='          # show it in a child environment

# Set a variable for this command only; the shell's LANG is unchanged
LANG=C sort names.txt

# Start a command with a deliberately small environment
env -i PATH="$PATH" sh -c 'printf "%s\n" "$PATH"'

Variable	Common purpose
HOME	Your home directory
USER	Login name; not set in every context
SHELL	Configured login shell, not necessarily the shell currently running
TERM	Terminal capability type
LANG	Default locale, affecting language, sorting, and character handling
PAGER	Preferred text pager for programs that honor it
EDITOR	Preferred text editor for programs that honor it

Inspect values safely with printf '%s\n' "$HOME", not with assumptions about their contents. Do not put passwords, API tokens, or other secrets in environment variables on shared systems: depending on the system and permissions, other processes or diagnostic tools may be able to expose them.

Startup Files and Interactive Customization

A shell reads startup files according to both the shell and whether it is a login or interactive shell. Bash commonly uses ~/.bashrc for interactive shells and one of ~/.bash_profile, ~/.bash_login, or ~/.profile for login shells. Zsh commonly uses ~/.zshrc; many POSIX shells use ~/.profile for login sessions. Exact behavior differs, and macOS has used Zsh as the default interactive login shell since Catalina while still providing other shells.

Put shell-specific syntax only in that shell's files. Aliases are usually disabled in non-interactive scripts, and functions are not automatically available to child shells. Keep scripts explicit: set needed variables, use full paths when justified, and name the interpreter in a shebang when relying on non-POSIX features.

Exit Status and “Command Not Found”

After a command, $? holds its exit status. Zero conventionally means success; nonzero means failure. Check it immediately, because the next command replaces it.

grep -q 'ready' status.txt
printf 'grep status: %s\n' "$?"

When the shell says “command not found,” diagnose methodically rather than installing a random package:

1. Check spelling and whether the command needs a path: command -V name and command -v name.
2. Inspect PATH one directory per line. Confirm the intended directory is present and ordered appropriately.
3. If you expected a local program, verify it exists and is executable: ls -l ./name; run it as ./name, not merely name.
4. Check whether documentation names a different utility on your OS, then use man -k or the system's package documentation to find the provider.
5. If it worked in another terminal, compare shell, login status, and relevant startup files. Do not copy another machine's PATH blindly.

Practice

1. Use man 1 printf and then command -V printf. Explain why their results may describe different implementations.
2. Run command -V cd, command -V echo, and command -V ls. Identify at least one shell built-in and one external command.
3. Create colour=blue, print it, export it, and confirm it appears in env. Open a new shell afterward and observe that a non-startup-file assignment did not persist.
4. Temporarily run LANG=C date, then run date normally. Compare the output without changing your permanent configuration.