How the Shell Finds Commands: PATH Explained

When you type ls and press Enter, how does the shell know where the ls program lives on disk? It searches the directories listed in your PATH environment variable, in order, until it finds an executable file with that name. PATH is one of the most important variables on your system — and the most common cause of “command not found” errors.

See your current PATH

echo $PATH
# typical output:
# /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/home/you/.local/bin

That’s a colon-separated list. The shell searches each directory in order — first match wins.

Find where a command actually lives

which ls            # /usr/bin/ls
which python3       # /usr/bin/python3
which docker        # /usr/bin/docker (or wherever)

# Show ALL matches in PATH (catches conflicts)
which -a python

# More info, including aliases and shell builtins
type ls
type cd

which only shows the first match. Use type -a or which -a to find conflicts (same command in multiple PATH directories).

Standard PATH directories

Directory	Purpose
/bin	essential commands (ls, cp, mv) — usable when only / is mounted
/sbin	essential admin commands (mount, fdisk)
/usr/bin	most user programs
/usr/sbin	most admin programs
/usr/local/bin	manually installed (not from package manager)
~/.local/bin	per-user installs (pip install –user, etc.)

Add a directory to PATH

Append (lowest priority — system commands win):

export PATH="$PATH:/opt/myapp/bin"

Prepend (highest priority — your override wins):

export PATH="/opt/myapp/bin:$PATH"

Make it permanent — add the line to ~/.bashrc (bash) or ~/.zshrc (zsh). The change takes effect for new shells, or run source ~/.bashrc to reload.

Run a command not in PATH

Use the full path or a relative path. The leading ./ matters:

/opt/tools/myscript.sh        # absolute path
./myscript.sh                  # relative path (same dir)

Why ./? For security. The current directory is NOT in PATH on Linux (unlike Windows), so a malicious file named ls in a folder you cd into won’t run when you type ls.

Common PATH problems

“command not found” but the file exists

• The directory containing it is not in PATH. Run echo $PATH to check.
• The file is not executable. Run chmod +x file.

Wrong version of a command runs

Two versions of Python or node in PATH; the first one wins. Check with which -a python. Reorder PATH so the right one comes first.

Sudo can’t find a command you can run

Sudo uses a restricted PATH for security. Use the full path with sudo:

sudo /home/you/.local/bin/myscript

What to learn next

PATH is one of many environment variables. The next node covers the broader system of environment variables and how to manage them.