C shell

Basic unix shell functionality implemented in C

User-defined interactive shell program that can create and manage new processes. The shell is able to create processes out of system programs like emacs, vi or any user-defined executable.

To compile:

make shell

To run:

./shell

Instructions

• <username@system_name:current_directory> is the format of the displayed prompt.

• The directory from which the shell is invoked is the home directory of the shell & is indicated by '~'.

• 'cd' to change directory.

• 'echo' to display a string of text.

• 'pwd' to display the present working directory.

• 'ls' to list directory contents with flags 'a' and 'l' for all content including hidden files & content displayed in long form respectively.

• '&' to designate a command as a background process.

• 'pinfo' displays process related information of the shell program. Usage: 'pinfo <process_id>'

• Appropriate messages are given to the user when a background process exits. For example, 'emacs with pid 456 exited normally'.

• 'remindme' displays a custom message to the user after a specified number of seconds. For example, 'remindme 1000 “Water the Plants”'.

• 'clock' command to display the current date and time after fixed intervals of seconds. For example, 'clock -t 5' yields:

  10 Sep 2020, 05:10:20
  10 Sep 2020, 05:10:25
  10 Sep 2020, 05:10:30

• Input-Output Redirection using '<', '>', and '>>'. For example, 'sort < lines.txt >> sortedlines.txt'

• Command redirection using pipes identified by '|'. For example, 'more file.txt | wc'

• 'setenv' to create if needed and assign values to shell environment variables. Initially, the shell inherits environment variables from its parent. Usage: 'setenv \ \'

• 'unsetenv' to destroy a shell environment variable. Usage: 'unsetenv <variable>'

• 'jobs' to print a list of all currently running jobs along with their process id and state in order of their times of creation.

• 'kjob' to send signals to particular jobs. Usage: 'kjob <job_id> <signal>'

• 'fg' to bring a running or a stopped background job with given job id to foreground. Usage: 'fg <job_id>'

• 'bg' to change status of a stopped background job to running. Usage: 'bg <jobNumber>'

• 'overkill' to kill all background processes.

• 'quit' to exit the shell.

• ctrl + Z changes the status of currently running job to 'stopped' and makes it a background process.

• ctrl + C sends a SIGINT signal to the current foreground job.

Implementation

• No popen, pclose, or system() calls were used.

• The user can type in any command, including './a.out' which starts a new process. The shell is able to execute the command or show the appropriate error message if the command cannot be executed.

• The user can type the command anywhere in the command line with any number of spaces or tabs.

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