29 Jan building a unix shell
This assignment asks you to build simple Unix shell and gives you in the process an opportunity to
demonstrate that you are mastering concurrency (including running external executables) and file
manipulation using the POSIX API. You may have seen similar assignments in other courses but
they are similar and not identical; make sure you follow the specifications herein, not something
Our shell repeatedly provides a prompt, accepts a command (possibly with arguments), and
executes it. Most commands are external executables, but some are internal.
The shell ignores completely its environment and reads a simple configuration file upon
startup instead. The file is named shconfig and resides in the current directory. It contains pairs
consisting in a variable name and a value for that variable, separated by at least one blank. The
two interesting lines in this file have the form
where v and h are strings representing positive integers. These variables are used by the internal command more. If these lines do not exist (or if the file is missing altogether), two such lines
must be appended to the configuration file (which must be created first if not already there) with
the values 40 for v and 75 for h. The configuration file may contain other lines, which should be
The following functionality should then be implemented:
All the typed in commands should be considered external except the commands more and
exit. Your shell should attempt to execute these commands. The search path for the external
commands contains only the directories /bin and /usr/bin (so any command outside these
directories should be ignored unless it is given by absolute path). If the command cannot
be found, then the shell should print an appropriate message; if the command produces an
error during its execution, then the shell should print yet another appropriate message (in
addition to whatever is printed by the command itself).
The arguments for the commands are provided in the same manner as in normal shells.
A command may be prefixed by the character & followed by a blank. If this is the case, then the
command is executed in the background, in the sense that the shell presents the prompt (and
accepts another command) before the completion of the previous command. Whenever the
background command terminates (with or without errors) the shell must print an appropriate
message, again in addition to whatever is printed by the command.
The command more must be implemented internally. This command receives one file name
as argument, and displays it to the standard output page by page. The number of columns
in a page and the number of characters in a column are taken from the configuration file and
are given by the value v of variable VSIZE and by the value h of variable HSIZE, respectively1
The command more displays v lines of the file given as argument and then awaits user input.
If the input is the character blank, then the next v lines are displayed; if the input is anything
else, then the command terminates. The command also terminates when it reaches the end
of the file. All the lines are displayed truncated to h characters if necessary (i.e., if they are
longer than h characters).
The command exit terminates the shell.
What to submit For this assignment you must use the POSIX interface presented in class for file
access rather than library functions.
The default target of your makefile must produces an executable named sshell and residing
in the root directory of your submission