The shell

sh is mcos’s command interpreter, after POSIX sh. It reads commands, expands their words and runs them. Only shell behavior lives in it: commands are programs in /bin, found through PATH, or named by a path, and the builtins are only what has to change the shell itself. The full reference is the sh page; this one is the tour.

The prompt

An interactive shell shows PS1, by default \w \$ : the working directory (with ~ for HOME unless HOME is /) and a $. The console’s shell is a login shell (sh -l), which runs /etc/profile and then /home/.shrc. The profile sets PATH, HOME, ENV and three aliases: ll (ls -alF), la (ls -a) and l (ls -lh --color=auto). Put your own aliases and variables in /home/.shrc.

Commands

$ ls /bin | grep s > names
$ echo "$HOME" ${USER:-nobody}
/home nobody
$ n=$(ls | head -n 1); echo $n
$ cat *.lua 2> /dev/null || echo none
$ sleep 5 &

The shell has words with quotes and backslashes, $ expansions ($NAME, ${NAME:-word}, $?, $$, $!, $#, $@), command substitution ($(...) and backticks), field splitting on IFS, and globs (*, ?, [...]). Commands are joined with |, &&, ||, ;, a new line and & (in the background). Redirections are <, >, >>, n>&m and n>&-.

The builtins are cd, pwd, exit, export, unset, set, ., source, alias, unalias, read, type, command, true, false and :.

What isn’t there yet: if, for, while, case, functions, here-documents and job control. A line is simple commands joined by the operators above. For logic, write the program in Lua: every mcos program is Lua.

Running a program

A word that names a file with a / is a path; any other is looked up in the directories of PATH. A Lua file runs directly, so a program you write in /home/hello.lua runs as /home/hello.lua (or lua /home/hello.lua). A first line starting with #! is skipped. A command’s exit status is $?: 127 for not found, 126 for found but not runnable, and 128 plus the signal’s number when a signal ended it (130 for Ctrl+C).

Line editing

At a terminal the shell reads its lines with readline, a library that works as GNU readline does and with bash’s usual bindings:

Tab completes the first word of a command to builtins, aliases and programs in PATH, and any other word to file names (directories end in /). One match is put in, followed by a space; several put in what they all start with, and a second Tab lists them. A line can be edited anywhere, across the rows it wraps onto. Keys typed ahead while a command starts are handed to that command, and echoed where the cursor is, as bash does.

While a command runs, Ctrl+C ends it (the shell carries on).

The tty

The terminal line is the kernel’s, a line discipline like Linux’s with its termios flags: icanon, echo, echoe, echoctl, isig, icrnl, onlcr and iutf8. stty shows and sets them (stty raw, stty sane, stty size). A program that wants every key turns the line raw, as the editor edit and readline do while you type.

Learning more

man shows a manual page; every program has one, and they are the same text as the Programs pages. lsdev lists devices, ps processes, dmesg the kernel’s log, free memory and uptime the time since boot.