ECS150 FQ20 April 3, 2020
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Project 1
Due April 13, 2020 at 11:59 PM
Early Extra Credit Due April 6, 2020 at 11:59PM
You will be working alone for this project. This specification is subject to change at anytime for
additional clarification. For this project, you will be implementing a fairly simple shell called
ashell. The shell must be able to execute applications, be able to setup pipes between executed
applications, redirect standard input/output from/to files for the executed applications, and
perform simple commands internally. You must provide a make file (not a cmake file) to
compile your shell. Your program must be written in C or C++. You may not use scanf, fscanf,
printf, fprintf, cin, cout, stringstream, or their derivatives; you must use read and write system
calls for all I/O. You may use the C++ containers. If you have any concerns if a standard library
function is allowed, ask before using it. Submit your project on Canvas as tgz file.
The shell must handle commands cd, ls, pwd, ff, and exit internally. Below are descriptions of
the commands:
cd [directory] – Changes the current working directory to specified directory. If the
optional directory is not specified, then the current working directory is changed to the directory
specified by the HOME environmental variable.
ls [directory] – Lists the files/directories in the directory specified, if no directory is
specified, it lists the contents of the current working directory. The order of files listed does not
matter, but the type and permissions must precede the file/directory name with one entry per line.
Below is an example listing
drwxr-xr-x .
drwxr-xr-x ..
-rwxr-xr-x ashell
-rwxr-xr-x main.cpp
pwd – Prints the current working directory name.
ff filename [directory] – Prints the path to all files matching the filename parameter
in the directory (or subdirectories) specified by the directory parameter. If no directory is
specified the current working directory is used. The order of the files listed does not matter.
Below is an example of a listing of ff main.cpp ECS150:
ECS150/Project1/main.cpp
ECS150/Project2/main.cpp
ECS150/main.cpp
exit – Exits the shell.
The shell needs to be able to handle up and down arrows to select through the history of
commands. Only the most recent 10 commands need to be stored in the history. The up arrow
must toggle to the previous command, down arrow to the next command. If the user attempts to
navigate beyond the end of the list the audible bell character '\a' (ASCII 0x07) must be output. In
ECS150 FQ20 April 3, 2020
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addition, if a user enters a backspace or delete and no characters exist on the command to delete,
the audible bell must be output. You will find the noncanmode.c program helpful converting
being able to read in a single character at a time. This will also be helpful in discovering the
VT100 escape codes for arrows. Remember that characters aren’t echoed out in non-canonical
mode.
The shell prompt must have the current working directory followed by the percent symbol. If the
full current working directory path is longer than 16 characters, then "/…" should be prepended
to the last level directory name. Below are examples:
/home/cjnitta%
/.../Project01%
A working example can be found on the CSIF at /home/cjnitta/ecs150/ashell.
You should avoid using existing source code as a primer that is currently available on the
Internet. You must specify in your readme file any sources of code that you have viewed to help
you complete this project. Any copied code snippets from online sources must have the URL
source in comments next to its use. All class projects will be submitted to MOSS to determine if
pairs of students have excessively collaborated with other pairs. Excessive collaboration, or
failure to list external code sources will result in the matter being transferred to Student Judicial
Affairs.
Early Extra Credit
Due April 6, 2020 at 11:59PM
You can receive extra credit by starting early on the project. Submit your project that can take in
user input, from the command line. It should be able to change directories with cd, output the
current directory, and handle exit commands. Up/down arrows should work for this as well. All
other commands should output the command followed by each argument separated by only a
single space. For example the command:
/home/cjnitta% ls -la foo
Should output:
Command: ls -la foo
A working example of the prompt only version can be found on the CSIF at
/home/cjnitta/ecs150/ashell-prompt-only.
Helpful Resources
System calls will be necessary for this project. You will definitely use the following system calls:
open() – Opens a file and potentially creates it if specified. See man page at
http://man7.org/linux/man-pages/man2/open.2.html
close() – Closes a file descriptor. See man page at http://man7.org/linux/manpages/man2/close.2.html
ECS150 FQ20 April 3, 2020
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read() – Reads data from a file descriptor (file, pipe, etc.). See man page at
http://man7.org/linux/man-pages/man2/read.2.html
write() – Writes data to a file descriptor (file, pipe, etc.). See man page at
http://man7.org/linux/man-pages/man2/write.2.html
pipe() – Creates a pipe. See man page at http://man7.org/linux/man-pages/man2/pipe.2.html
dup2() – Duplicates a file descriptor (note use dup2 instead of dup it is easier). See man page
at http://man7.org/linux/man-pages/man2/dup.2.html
fork() – Makes a copy of a process. See man page at http://man7.org/linux/manpages/man2/fork.2.html
execvp() – Replaces the calling process with a new program (note execvp may be the
easiest to use but there are many derivatives of exec). See man page at
http://man7.org/linux/man-pages/man3/exec.3.html
wait() – Waits for a child to terminate. See man page at http://man7.org/linux/manpages/man2/wait.2.html
getcwd() – Gets the current working directory. See man page at http://man7.org/linux/manpages/man2/getcwd.2.html
chdir() – Changes the current working directory. See man page at http://man7.org/linux/manpages/man2/chdir.2.html
opendir() – Opens a directory for reading. See man page at http://man7.org/linux/manpages/man3/opendir.3.html
readdir() – Reads a directory entry in from a directory opened with opendir. See man page at
http://man7.org/linux/man-pages/man3/readdir.3.html
closedir() – Closes a directory opened with opendir. See man page at
http://man7.org/linux/man-pages/man3/opendir.3.html
stat() – Gets the information about a file. See man page at http://man7.org/linux/manpages/man2/stat.2.html
getenv() – Gets an environmental variable, helpful for getting HOME directory. See man
page at http://man7.org/linux/man-pages/man3/getenv.3.html
You may find getpid() http://man7.org/linux/man-pages/man2/getpid.2.html
ECS150 FQ20 April 3, 2020
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helpful in determining which process is running what when outputting. During your debugging
you will probably want to output the pid of the process with each output.
Overview of simple shell command
1. Print prompt
2. Wait for command input
3. Parse into command and arguments
4. fork the shell
5. Child and parent determine who they are by fork return
6. Parent waits while child executes with call to wait
7. Child calls exec to become the command
8. New exec process runs and finally terminates
9. Parent shell returns to step 1
Internal commands and forking
Some of the internal commands you will not want the child to handle, while others you will. The
parent should handle cd and exit, the reason for this is that if the child handles cd the
directory will only be changed for the child, and not the parent. Also, if the child handles exit,
then nothing will have actually happened. The commands that the child should handle are ls,
pwd, and ff. The reason that the child should handle these is that the output of them might be
redirected or piped to another process. The parent shell output should not be redirected, and
therefore the child needs to handle these commands.
Redirecting overview
Redirecting opens a file and redirects either stdin or stdout to it. Here are some things to
remember when trying to implement redirecting.
• You will probably want to open the file after forking because the parent will not
necessarily need access to it
• You will want to set the mode for the file if you are creating it, failure to do so may not
grant read permission to the file afterward.
• dup2 should be used to redirect the file to either stdin or stdout
Piping overview
Piping is similar to redirecting but uses a pipe and multiple children. Piping between two
children processes requires multiple calls to fork, creating a pipe, and redirection of stdin/stdout
before exec. Here are some things to remember when trying to implement piping.
• Pipe must be created before a fork (remember fork clones, so if pipe is done afterward the
child or parent doesn’t know the pipe created)
• dup2 should be used to redirect an end of the pipe to either stdin or stdout (each pipe has
a read and a write end)
• Don’t forget to close the pipe ends that are no longer needed after calling dup2 before
calling exec. An open write end can make a process hang expecting that input is still
possible.
ECS150 FQ20 April 3, 2020
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