shan18 / Simple-Shell

A simplified version of the Unix shell.

Simple Shell

This is a very simplified version of the Unix shell.

Compiling and Usage

To compile the project, run the command

$ make

To start using the shell, run the executable file generated after compiling

$ ./shell

Supported Features

The shell supports the following features

Built-in Commands

The shell contains four built-in commands

• cd [dir]: For changing directories.
• exit: To terminate the shell.
• jobs: Prints a list of currently suspended jobs to STDOUT.
• fg [index]: Resumes a job in the foreground.

More details about the jobs and the fg commands are given below.

Running External Programs

An external program can be specified by either an absolute path, a relative path, or base name only.

If only the base name of the command is specified, the shell searches for the program under /bin and /usr/bin (in such order). For example, if the user enters ls, the shell first tries /bin/ls, if that fails then /usr/bin/ls is tried.

I/O Redirection

Input Redirection

The standard input (STDIN) can be redirected using <. For example:

[user dir]$ cat < input.txt

Output Redirection

The standard output (STDOUT) can be redirected using > and >>. If the output file already exists, redirecting with > will overwrite the file, whereas >> will append to the existing file. For example:

[user dir]$ ls -l > output.txt
[user dir]$ ls -l >> output.txt

Pipe

A pipe (|) can be used to connect the standard output of the first program to the standard input of the second program. For example:

[user dir]$ cat shell.c | grep main | less

Suspending and Resuming Processes

If a child process (job) has been suspended via Ctrl-Z or a SIGTSTP or SIGSTOP signal, the shell will keep a record of it so that the child can be resumed later.

To check the list of currently suspended jobs, the jobs command can be used. It prints the information in the following format: [index] command. For example

[user dir]$ jobs
[1] cat
[2] top | cat
[3] cat > output.txt
[user dir]$ _

The list printed above is sorted by the time each job is suspended (oldest first).

To resume a suspended job, the fg command can be used. It takes exactly one argument: the job index, which is the number in the bracket printed by the jobs command. For example

[user dir]$ jobs
[1] cat
[2] top | cat
[3] cat > output.txt
[user dir]$ fg 2

This command would resume top | cat in the foreground.

Contact/Getting Help

If you need any help or want to report a bug, raise an issue in the repo.