Smallsh (Small Shell)

A custom, interactive shell implementing a subset of well-known UNIX shell features. Written in C.

Features

• Single-line shell commands
• Built-in exit, cd, and status commands
• Execution of other PATH binaries
• I/O redirection
• Foreground and background processes
• Custom handlers for SIGINT and SIGTSTP signals

Limitations

• Maximum of 2048 characters in command, including variable expansion of $$
• Maximum of 512 arguments, including the command name
• Maximum of 500 concurrent background processes

Compilation

• Compile using GCC with the -std=gnu99 flag for C99 standards with GNU extensions
• Full command: gcc -O3 -std=gnu99 -Wall -Wextra -Wpedantic -Werror smallsh.c -o smallsh
• Alternatively, run make using the provided Makefile

Usage

• Start the shell by running ./smallsh
• Once started, the shell will display : as its command prompt
• Smallsh supports commands in the following format:
  • command [arg1 arg2 ...] [< input_file] [> output_file] [&]
    • ...where items in square brackets are optional
    • command is the name of the command or PATH binary you wish to execute
    • < will redirect input from stdin to the specified input_file
    • > will redirect output from stdout to the specified output_file
    • & is a flag to run the process in the background
• Run cd [path] to change the working directory. Supports both relative and absolute paths.
• Run status to display the exit value or termination signal of the last terminated process
• Run exit to kill any background processes and terminate the shell
• Send a SIGINT signal (e.g. Ctrl + C) to terminate any foreground process
• Send a SIGTSTP signal (e.g. Ctrl + Z) to enter foreground-only mode, in which the & flag will be ignored and all processes will run in the foreground
  • Send another SIGTSTP signal to exit foreground-only mode

Example

$ ./smallsh
: ls
junk  smallsh  smallsh.c
: ls > junk
: status
exit value 0
: cat junk
junk
smallsh
smallsh.c
: wc < junk > junk2
: wc < junk
 3  3 23
: test -f badfile
: status
exit value 1
: wc < badfile
cannot open badfile for input
: status
exit value 1
: badfile
badfile: No such file or directory
: sleep 5
^Cterminated by signal 2
: status &
terminated by signal 2
: sleep 15 &
background pid is 4338
: ps
  PID TTY          TIME CMD
 4236 pts/0    00:00:00 bash
 4328 pts/0    00:00:00 smallsh
 4338 pts/0    00:00:00 sleep
 4339 pts/0    00:00:00 ps
:
: # that was a blank command line, this is a comment
:
background pid 4338 is done: exit value 0
: # the background sleep finally finished
: sleep 30 &
background pid is 4340
: kill -15 4340
background pid 4340 is done: terminated by signal 15
: pwd
/home/rngolam/smallsh
: cd
: pwd
/home/rngolam
: echo 4328
4328
: echo $$
4328
: ^C^Z
Entering foreground-only mode (& is now ignored)
: date
Sat May  7 21:11:11 PDT 2022
: sleep 5 &
: date
Sat May  7 21:11:16 PDT 2022
: ^Z
Exiting foreground-only mode
: date
Sat May  7 21:12:33 PDT 2022
: sleep 5 &
background pid is 4350
: date
Sat May  7 21:12:33 PDT 2022
: exit
$