Operating systems 1 -- 2009-2010 -- info.uvt.ro/Project 1

Quick links:

• front;
• laboratories: agenda, 1, 2, 3, 4, evaluation, tools, references, repository;
• projects: 1, 2, repository;

Description

The official project description should also be read from profesor Florin Fortiş. Please also consult the references at the end of this page.

Implement a C application that implements a basic shell that presents a prompter to the user and allows him to enter certain commands (the commands with their syntax and semantics are described in the next section);

The shell should implement the following basic loop:

• write the prompter to the user;
• reads the user command;
• parses and validates the user command;
  • in case there is a syntax or semantic error a meaningfull message should be printed to the user;
• executes the user command;
• writes a meaningfull success or failure message for the user;
• the loop should terminate when:
  • the user enters exit command; (this is the normal way to close the shell);
  • there is an error while reading from stdin (this should be treated as an error, and a meaningfull message should be printed);
  • there is an end-of-file encountered in stdin (this should be treated as a warning, but acceptable);

Syntax

Rules:

• each command should be written on its own line:
  • commands cannot span multiple lines;
  • multiple commands cannot be written on the same line;
• each command is composed of one or more tokens separated by one or more space or tab characters;
• each token is:
  • either a sequence of one or more characters except space or tab;
  • either a sequence of one or more characters (including space or tab), but enclosed inside double or single quotes; (the quotes are not part of the token, and the quotes character used cannot appear inside the token);
• a line whose first non-space (or tab) character is # should be treated as a comment and ignored;

Examples:

• comment lines:

# this is a comment line
   # this is another comment line
##### again a comment line

• simple command without quoted tokens (observe how the whitespace does not matter):

mkdir /tmp/test-folder
   rmdir    /tmp/test-folder
echo test
exit

• a command with quoted tokens:

# the text enclosed in single quotes is one token (including those double quote characters)
echo ' aaa bb c """"" '

Commands

List of commands

generic commands	flags	points
help		R
version		R
exit		R
file commands	flags	points	man page
cat	-b, -E, -n, -s	3	cat.1
tac	-b -s	3	tac.1
head	-c, -n, -q, -v	3	head.1
tail	-c, -n, -q, -v	3	tail.1
sort	-i, -r, -d	4	sort.1
uniq	-i, -d, -u	4	uniq.1
nl	-s -d	2	nl.1
wc	-c, -w, -l, -L	3	wc.1
expand	-t, -i	3	expand.1
unexpand	-t, -i, -a	3	unexpand.1
tee	-a	3	tee.1
file system commands	flags	points	man page
cd		2	bash.1 (see cd comamnd)
pwd		1	bash.1 (see pwd comamnd)
cp	-i, -R, -t, -v	5	cp.1
mv	-i, -t, -s	4	mv.1
rm	-i, -R -v	4	rm.1
mkdir	-m, -p, -v	2	mkdir.1
rmdir	-p, -v	2	rmdir.1
ls	-l, -s, -a, -F	5	ls.1
dir		3	dir.1
chmod	-v, -R	5	chmod.1
chown	-v, -R	5	chown.1
chgrp	-v, -R	5	chgrp.1
dirname		2	dirname.1
basename		2	basename.1
environment commands	flags	points	man page
printenv		2	printenv.1
env	-u	3	env.1
miscellaneous commands	flags	points	man page
yes		1	yes.1
date	-d, -f, -s, +<format>	4	date.1

Generic requirements

• each command shall have understand the -h (help flag) -U (user flag) that should provide a short description and usage about the command (-h), or the implementor (-U);

help and version

• the help command shall provide a list of available commands and a short description for each one;
• the version command shall print the version, author and other informations about the implementation;

exit

• should cause the shell to stop asking for new commands and exit;
• example:

exit

echo

• should accept any number of arguments (including none), and display them separated by space;
• example:

echo 'a bb cc' cc d "ee ff"

• • output:

a bb cc cc d ee ff

echoenv

• should take only one argument that represents the name of an environment variable, and print it to standard output:
• example:

echoenv USER

• • output:

ciprian

• example in case of undefined variable:

echoenv NO_SUCH_VARIABLE

• • output (to standard error):

[ee] the environment variable `NO_SUCH_VARIABLE` is undefined;

• references:
  • getenv.3;

cd, and pwd

• cd should be used to change the current directory to the one pointed by the single argument given;
• pwd should print the current working directory;
• example 1 (both command (prefixed by $, and output (no prefix)):

$ pwd
/home/ciprian

• example 2 (both command and output):

$ cd /tmp
$ pwd
/tmp

• references:
  • chdir.2;
  • getcwd.3;

mkdir, and rmdir

• both take any number of arguments (at least one) and either create or remove the designated folder (rmdir should work only in case of empty directories);
• examples:

$ mkdir /tmp
[ee] the folder `/tmp` already exists;
$ mkdir /tmp/a /tmp/b /tmp/c
$ rmdir /tmp/a /tmp/b
$ rmdir /tmp/c
$ rmdir /tmp/x
[ee] the folder `/tmp/x` does not exist;

touch, link, and unlink

• touch creates the designated file in case it does not exist (it accepts more arguments, at least one);
• link takes two arguments (the source and destination) and makes a hard link of the source to destination;
• unlink takes any number of arguments (at least one) and deletes them;
• examples:

$ touch /tmp/a /tmp/b
$ link /tmp/b /tmp/c
$ unlink /tmp/c

cat, head and tail

• cat takes at least one argument and displays the content of each file one after another;
• head takes only one argument and displays the first 10 lines from it;
• tail is like head but displays only the last 10 lines;
• examples:

$ cat /etc/fstab /etc/services
... the content of /etc/fstab
... the content of /etc/services

• head and tail could also accept the -n option, followed by a number and should display that many first or last lines;
• examples:

$ head -n 20 /etc/services
... the first 20 lines from /etc/services

ls

• takes exactly one argument, the name of a folder, and should display the files found inside; (by default the folders ., .., and any file / folder starting . should not be displayed;)
• as modifiers (flags) we have:
  • -a -- should display even files starting with . (except the . and .. folders);
  • -l -- should display after each file the type, owner (uid), group (gid) and size;
• the contents should be sorted alphabeticaly;
• examples:

$ ls /
dev
etc
tmp
var
...
$ ls -a /home/ciprian
.bashrc
.bash_login
...
desktop
...
$ ls -l /boot
grub folder 0 0 4096
vmlinuz file 0 0 13341253

cp, and mv

• cp takes exactly two arguments: the source and destination, and it should copy the content of the source file as the content of the destination file;
• if the destination file already exists it should be treated as an error;
• if -f flag is present the copy is forced (even if the destination already exists);
• mv is the same as cp, but instead of copying it moves the file;
• mv should not be implemented by using the rename POSIX function (or by link and unlink); instead a new file must be created, filled with the contents of the old one; and then the old file should be deleted;

Notes

Mandatory observations:

• all messages that are meant for the user to see (informations, warnings, errors, etc.) should be printed to stderr;
• all text that is part of the command actual output (like in the case of ls) should be printed to stdout;
• stdin should be only used to read user commands;

Optional observations:

• in case of messages for the users it would be nice to prefix the text with an indicator of the nature / gravity of the message; for example for informative messages (like success) you could prepend one of the following prefixes: INFO:, or [ii], etc. (a space would be nice between the prefix and the actual output);

References

• from me:
  • Operating systems 1 -- 2009-2010 -- info.uvt.ro/Laboratory 2 -- the POSIX functions used there for file system management;
  • Operating systems 1 -- 2009-2010 -- info.uvt.ro/Laboratory 3 -- the additional readline and getopt helper functions;
• from Florin Fortiş:
  • Sample implementations;
  • Using C Tools;
  • Copying stdin to stdout;
• from Silviu Panica:
  • SO1 ENG:
    • Laboratory 4 -- building process;
    • Laboratory 5 -- shell skeleton and readline;
    • Laboratory 6 -- getopt, environment variables, directories;

Submission

Please consult the repository page.