5-shell

5-shell: Implement a Small UNIX Shell

Overview and Goal

The goal of this task is to write a simple but functional UNIX shell in C. It must:

• Print a shell prompt ($)
• Read multiple commands from standard input and execute them
• Support shell variables and expansion
• Support the variable $? to print out the return code of the last program
• Run programs in the background if desired
• Support redirection of standard input, output and error
• Support the pipeline operator (|)

See expected output below for details.

Important Notes

• Executing any real shell (e.g. /bin/bash) through exec(), popen() or any other means is not a valid solution. All of the functionality must be implemented by yourself in shell.c!
• Do not use isatty() to check if your shell is running in a terminal (e.g. to display a prompt only in this case). The CI environment is not using a terminal.
• It is recommended to use fflush() to flush the output streams (otherwise, e.g. CI tests may not work properly)

Expected Output

Print a shell prompt ($) when ready

...
$

Read multiple commands from standard input and execute them

$ echo Hello
Hello
$ expr 1 + 3
4

Support shell variables and expansion

$ A=5
$ echo $A
5
$ echo ${A}BC
5BC
$ A=$A$A
echo $A
55

Support checking the return code of the last program using $?

$ true
$ echo $?
0
$ false
$ echo $?
1

Support running programs in the background with &

$ firefox &
$

After this, Firefox should be running and the shell accepting commands again.

Support redirection of stdin, stdout and stderr with >, < and 2>

$ echo Hello > test.txt
$ cat - < test.txt
Hello

Support the pipeline operator |

$ echo Hello | tr l L | tr o ?
HeLL?

Evaluation

Total points possible: 7

Minimum number of points required to pass: 5

Points can be obtained as follows:

• 1 Point: Correct display of shell prompt
• 1 Point: Read and execute multiple commands
• 1 Point: Support shell variables and expansion
• 1 Point: Support $? for displaying the return code of the last program
• 1 Point: Support & for running programs in the background
• 1 Point: Support redirection of stdin, stdout and stderr
• 1 Point: Support pipes with |

Push the Result to GitLab

To test your code and finally submit your task, your changes must be committed and pushed to GitLab:

$ git add shell.c
....
$ git commit -m "solution for task 5"
....
$ git push origin master

You may make as many commits as you want; when grading the task, the latest commit at that point in time will be used.

Get Feedback

You can inspect the results of the automated tests run by the GitLab CI (https://gitlab.ti.bfh.ch). For this, go to "CI/CD > Jobs" in your project. In the output of the test job, the amount of points given will also be indicated.