make vsh
Run ./build/vsh from the source directory.
vsh is a shell with lots not a lot of features.
Multiple ';' separated commands can be executed in a single line.
Execute a file in the system specified by the absolute path or executable name specified as the first input argument, and pass the remaining arguments as input to the executable thus launched. If the specified filename does not contain a slash character, the file is sought in the colon-separated list of directory pathnames specified in the PATH environment variable. These files may be executed as a foreground or a background process depending on the input arguments.
A system command by default will execute on the foreground. The shell will wait until the execution of this process is completed before resuming operation.
A system command can be run in the background by appending a & to the
input command. The shell will spawn the process in a new, isolated process
group and will not wait for it to complete.
The Process ID of the new background process will be printed on successful
spawning, which can be used later to fetch information about the process.
Once a background process exits, the shell will alert the user about its
termination with the necessary info.
The Up and Down arrow keys can be used to cycle through the shell's
command history for easier repeated execution.
The key combination Ctrl+D can be used to exit the shell.
The key combination Ctrl+L can be used to clear the screen.
A list of commands built-in to the shell whose functions are impossible or inconvenient to implement by external executables.
The output of a process can be redirected to files using the symbols '>' (truncate & write) and '>>' (append), while input can be redirected to processes using the symbol '<'. Both input and output redirection can be used simultaneously.
The output of a foreground process can be redirected into another process as its input using the symbol '|' between two commands. Multiple commands can be piped together. Piping and redirection can be used in tandem.
The key combination Ctrl+C can be used to send a SIGINT, which is caught by the shell
and directed to the foreground process for it to handle.
The key combination Ctrl+Z can be used to send a SIGTSTP, which is caught by the shell
and causes the current foreground process to be suspended (stopped and sent to the background).
Usage: cd <path_to_directory>
Changes the current working directory to the absolute/relative path
specified by the first input argument. In case of no arguments, it
defaults to the shell's home.
Supports the following wildcards:
~: path to shell's home directory-: path to previous directory.: path to current directory..: path to parent directory
Usage: pwd
Displays the current working directory of the shell.
Usage: echo <string>
Echoes the line of text taken as user input onto stdout.
Does not support any options.
Does not support multi-line strings, quoted strings, escape sequences,
or environment variables.
A list of custom-implemented or user-defined shell commands.
Usage: ls [options] <path_to_directory_1> <path_to_directory_2> ...
Lists contents of the directories specified by input arguments. In case of no arguments, it defaults to contents of the current working directory. Supports the following options:
-a: do not ignore entries starting with .-l: use a long listing format Supports all the wildcards ascd.
Usage: pinfo <pid>
Prints the process-related information of the process specified by the first input argument. In case of no arguments, it defaults to the current shell's process. Gives the following info:
- Process ID
- Process Status
- Virtual Memory
- Path to Executable
Usage: repeat <number_of_times> <command>
Repeats the command specified by the second input argument sequentially as many times as specified by the first input argument.
Usage: history <number_of_entries>
Prints the latest number of entries from the shell's command history as
specified by the first input argument. In case of no arguments, prints all
the commands stored in history.
Stores upto 20 of the latest commands, and ignores consecutively repeated ones.
Commands are tracked across all sessions of the shell.
Usage: jobs [options]
Prints a list of all currently running background processes spawned by the shell in alphabetical order of the command name, along with their job number, process ID, and their state, which can either be 'Running' or 'Stopped'. Supports the following options:
-r: prints only running processes-s: prints only stopped processes
Usage: sig <job_id> <signal_number>
Takes the job number of a running job and sends the signal corresponding to the input signal number to the process.
Usage: fg <job_id>
Brings the running or stopped background job corresponding to the input job number
to the foreground and changes its state to running.
Throws an error if no job with the corresponding number exists.
Usage: bg <job_id>
Changes the state of a stopped background job to running (in the background).
Throws an error if no job with the corresponding number exists.
A list of additional commands added for convenience of the shell user.
Usage: exit
Exits the shell, and terminates all of its running background processes.
Usage: clear
Clears the terminal screen.
A few assumptions have been made that impact the performance and output of the shell as follows:
- The input to the shell may not exceed 2048 characters.
- The username of the current shell user may not exceed 256 characters.
- The hostname of the current shell user may not exceed 256 characters.
- The shell has access to retrieve the host user's username and hostname.
- The shell has permission to read the current working directory in the directories it will be run in.
- The maximum length of a path does not exceed 4096 characters.
- The maximum length of a process's
statfile (under/proc/<id>/stat) is 1024. This is necessary because the size of files under/proccan not be determined beforehand. - 1 month = 30.44 days = 2629743 seconds. This determines whether a file is older/newer than 6 months and conditionally displays the year or hours and minutes in long listing format.
- The shell user has read and write access to
/tmp. This is necessary to manage the history file. - Only foreground processes will be pipelined.
- Exactly one argument will be provided to the redirection operator that points to the input/output file.