The shell startup files and PATH

Lesson 69 left a question open: if variables die with the shell, who sets PATH, HOME and PS1 every time you log in? The answer is a handful of plain shell scripts that the shell runs on itself at startup. Which ones it reads depends on what kind of shell it is.

Login shells and /etc/profile

A login shell is the first shell of a session: the one started by login, by SSH, or here by init on each console. You can spot it by the dash in front of its name: echo $0 prints -/bin/sh. A login shell first runs /etc/profile, the system-wide file the administrator controls, and then ~/.profile, your personal one, if it exists. An interactive shell additionally runs the file named in the variable ENV, if it is set (ash's equivalent of bash's ~/.bashrc). A non-interactive shell, such as one running a script, reads none of them: scripts inherit the environment and nothing else.

~% echo $0
-/bin/sh
~% cat /etc/profile
export PATH=/bin:/sbin:/usr/bin:/usr/sbin
...
export PAGER='/bin/more '
export EDITOR='/bin/vi'

# Source configuration files from /etc/profile.d
for i in /etc/profile.d/*.sh ; do
	if [ -r "$i" ]; then
		. $i
	fi
	unset i
done
...
PS1='\W% '
HOME='/root/'
cd ~

The lab's /etc/profile is a good specimen. It exports PATH, PAGER and EDITOR, runs every *.sh file in /etc/profile.d/ (a drop-in directory, so packages can add settings without editing the main file), mounts filesystems, and finally sets the \W% prompt and the HOME with a trailing slash that you met in lesson 69.

Testing a change

~% echo 'export LAB70=yes' >> ~/.profile
~% echo "[$LAB70]"
[]
~% sh -l -c 'echo "login shell: [$LAB70]"'
login shell: [yes]
~% . ~/.profile
~% echo "[$LAB70]"
[yes]

Editing ~/.profile changes nothing in the shell you are typing in: it is only read at startup. Two ways to try it. . FILE (or source FILE) *sources* the file: runs it inside the current shell, so its exports and cds stick. sh -l starts a fresh login shell that reads the startup files from scratch, the honest test of what your next login will see. Do not run it as sh ~/.profile: that executes it in a child shell, and every setting dies with the child. And be careful with /etc/profile: a syntax error there breaks every login, so test with sh -l before you close your working session.

PATH: how a name becomes a program

~% echo $PATH
/bin:/sbin:/usr/bin:/usr/sbin
~% which ls
/bin/ls
~% type cd
cd is a shell builtin
~% command -v ls
/bin/ls
~% sysinfo
-/bin/sh: sysinfo: not found
~% PATH="$PATH:/root/bin"
~% sysinfo
sysinfo-ok

When you type a name without a slash, the shell first checks its builtins and functions, then tries each directory of PATH, left to right, and runs the first match; exit status 127 means none matched. A name containing a slash, such as ./script or /bin/ls, skips the search entirely. The current directory is deliberately *not* in PATH, so a malicious file called ls in some folder never runs by accident. which prints the file PATH would find, command -v does the same and also knows builtins, and type says what the name is. To add your own directory, append it with PATH="$PATH:/root/bin" (appending lets system commands keep priority) and put that line, with export, in ~/.profile to keep it. Lesson 93 builds on this to install your own commands.

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