smallsh

Shell program with built-in commands. Implements a subset of features of well-known shells, such as bash.

Features

1. Provides a prompt for running commands
2. Handles blank lines and comments, which are lines beginning with the # character
3. Provides expansion for the variable $$ - Replaces any instance of '$$' in a command with the process ID of shell
4. Executes 3 commands exit, cd, and status via code built into the shell
5. Executes other commands by creating new processes using a function from the exec family of functions
6. Supports bash style input and output redirection with '<' and '>'
7. Supports running commands in foreground and background processes
8. Includes custom handlers for 2 signals, SIGINT (CTRL+C) and SIGTSTP (CTRL+Z)
   1. SIGINT terminates foreground child processes and prints out PID of the process and the signal that killed it, SIGINT is ignored by shell and background processes
   2. SIGTSTP flips shell into 'foreground only mode' where '&' is ignored until shell receives SIGTSTP again, SIGTSTP is ignored by all child processes (foreground and background)

Compilation and execution

Please compile with command:

gcc --std=gnu99 main.c -o smallsh

Execute:

./smallsh

Sample Execution of the Program

$ 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 4923

: ps
  PID TTY          TIME CMD
 4923 pts/0    00:00:00 sleep
 4564 pts/0    00:00:03 bash
 4867 pts/0    00:01:32 smallsh
 4927 pts/0    00:00:00 ps
:
: # that was a blank command line, this is a comment line
:
background pid 4923 is done: exit value 0

: # the background sleep finally finished
: sleep 30 &
background pid is 4941

: kill -15 4941
background pid 4941 is done: terminated by signal 15

: pwd
/nfs/stak/users/chaudhrn/CS344/prog3

: cd
: pwd
/nfs/stak/users/chaudhrn

: cd CS344
: pwd
/nfs/stak/users/chaudhrn/CS344

: echo 4867
4867

: echo $$
4867

: ^C^Z
Entering foreground-only mode (& is now ignored)
: date
 Mon Jan  2 11:24:33 PST 2017
 
: sleep 5 &
: date
 Mon Jan  2 11:24:38 PST 2017
 
: ^Z
Exiting foreground-only mode

: date
 Mon Jan  2 11:24:39 PST 2017
 
: sleep 5 &
background pid is 4963

: date
 Mon Jan 2 11:24:39 PST 2017
 
: exit
$