Shell

Introduction

This a Unix shell similar to sh, bash and zsh. This implementation is not based on (or exploit) another shell. For example, system(3).

Implementation Details

• This shell could run all simple commands.
• This shell supports the cd and exit following built-in command
• It runs sequences of 3 commands or more properly.
• It runs pipes of 2 simple commands properly.
• This shell can pass all valgrind checks, as well as gcc -Wall -Wextra.
• It will print an error message on the standard output when an API function fails.
• This shell can handle interrupts such as Ctrl+C from users properly, but regular commands will still be interrupted when the user enters Ctrl+C.
• This implementation runs pipes of more than 2 parts consisting of sequences or pipes of simple commands.
• This shell supports redirections, detached commands and executing commands in a sub-shell.
• It supports the set and unset built-ins for managing environment variables.
• It can parse the PS1 correctly and support hostname, user name and current path. ( see PROMPTING in the bash(1) manual page)
• This shell supports simple job control: Ctrl+Z to suspend a command group, bg to continue a job in the background, and fg to recall a job to the foreground.

Example commands

## simple commands:
ls
sleep 5   # must not show the prompt too early

## simple commands, with built-ins:
mkdir t
cd t
/bin/pwd  # must show the new path
exit 42   # terminate with code

## sequences:
echo hello; echo world # must print in this order
exit 0; echo fail  # must not print "fail"

## pipes:
ls | grep t
ls | more    # must not show prompt too early
ls | sleep 5 # must not print anything, then wait
sleep 5 | ls # must show listing then wait
ls /usr/lib | grep net | cut -d. -f1 | sort -u

## redirects:
>dl1 ls /bin; <dl1 wc -l
>dl2 ls /usr/bin; >>dl1 cat dl2 # append
<dl2 wc -l; <dl1 wc -l # show the sum
>dl3 2>&1 find /var/. # errors redirected

## detached commands:
sleep 5 &  # print prompt early
{ sleep 1; echo hello }& echo world; sleep 3 # invert output

## sub-shell:
( exit 0 ) # top shell does *not* terminate
cd /tmp; /bin/pwd; ( cd /bin ); /bin/pwd # "/tmp" twice

## environment variables
set hello=world; env | grep hello # prints "hello=world"
(set top=down); env | grep top # does not print "top=down"

# custom PATH handling
mkdir /tmp/hai; touch /tmp/hai/waa; chmod +x /tmp/hai/waa
set PATH=/tmp/hai; waa # OK
unset PATH; waa # execvp() reports failure

Syntax of built-ins

Built-in: cd <path> : Change the current directory to become the directory specify in the argument. Your shell does not need to support the syntax cd without arguments like Bash does.

Built-in: exit <code> : Terminate the current shell process using the specified numeric code. Your shell does not need to support the syntax exit without arguments like Bash does.

Built-in (advanced): set <var>=<value> : Set the specified environment variable. Your shell does not need to support the syntax set without arguments like Bash does.

Built-in (advanced): unset <var> (optional) : Unset the specified environment variable.

Error handling

This shell might encounter two types of error:

• When an API function called by the shell fails, for example execvp(2) fails to find an executable program For these errors, this shell will print a usefu error message on its standard error. The helpe function perror(3) is used for this purpose.
• When a command launched by the shell exits with a non-zero status code, or a built-in command encounters an error. For these errors, this shell will print a useful indicative message but this will not be tested.

Notes

1. A shell usually supports redirections on all places of a simple command; ls > foo and >foo ls are normally equivalent. However, this shell only supports >foo ls.
2. Within a 'pipe' construction, all parts should be forked, even if they only contain built-in commands. This keeps the implementation easier.

exit 42 # closes the shell
exit 42 | sleep 1  # exit in sub-shell, main shell remains

cd /tmp # changes the directory
cd /tmp | sleep 1  # change directory in sub-shell
                   # main shell does not

Execution

1. install dependencies on Ubuntu using the following command:

   sudo apt install build-essential python python-pexpect libreadline-dev flex valgrind

2. Use make check to run tests.