Starting from:
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CSCE313-Assignment 3 Linux Shell Solved
For this assignment, you are to design a simple shell which implements a subset of the functionality of the Bourne Again Shell (Bash). The requirements for your shell are as follows:
• Continually prompt for textual user input on a command line.
• Parse user input according the provided grammar (see below)
• When a user enters a well formed command, execute it in the same way as a shell. You must use the commands fork and exec to accomplish this. You may NOT use the C++ system() command.
• Allow users to pipe the standard output from one command to the input of another an arbitrary number of times.
• Support input redirection from a file and output redirection to a file.
• Allow users to specify whether the process will run in the background or foreground using an ’&’. Backgrounding processes should not result in the creation of zombie processes. (Commands to run in the foreground do not have an ’&’, and commands that run in the background do)
• Print a custom prompt which supports printing the current directory, user name, current date, and current time.
• (Bonus) Allowing $-sign expansion. See the last sample command in the list of commands for example.
Write a report describing your design for piping, redirection and other special techniques that you are unique to your implementation. No need to restate the obvious.
Figure #4: Simple Shell Grammar
valid string = unix command || unix command AMP || special command
unix command = command name ARGS || unix command REDIRECTION filename ||
unix command PIPE unix command
special command = cd DIRECTORY || exit command name = any valid executable/interpreted file name
AMP = &
ARG = string
ARGS = ARG ARGS || ARG
DIRECTORY = absolute path || relative path
PIPE = |
REDIRECTION = < ||
Figure #4: Some Sample Commands
echo Here starts our shell ...
echo −e ‘‘<<<<< This message contains a line feed \n’’ echo ‘‘<<<<< Start to exercise pipe ’’
echo ‘‘<<<<< IO redirection ’’ ps test.txt
grep pts < test.txt pwd pwd.txt mv pwd.txt newpwd.txt cat newpwd.txt
echo ‘‘ 1 pipe ’’ ps
echo ‘‘ 2 pipes ’’
psps −−aa || awk ’/pts/[0awk ’/pts/[0−−9]9]//{{print $1print $2}}’’ || tailtail
echo ‘‘<<<<< 3 pipes ’’
lslsls −−−l /proc/sysl /proc/sysl /proc/sys ||| awk ’awk ’awk ’{{{print $9print $8$9print $8}}’’ }’|||headsorthead−−−r310|||headsortsort −−r5
echo ‘‘<<<<< 4 pipes with I/O redirection ’’
awk ’cat output.txtls −l /proc/sys{print $8$9}’ test.txt< test.txt | head −10 | head −8 | head −7 | sort output.txt
echo ‘‘<<<<< Background process (ps after some time to make sure the bg’s are not defunct) ’’
dd if=/dev/zero of=/dev/null bs=1024 count=10485760 &
sleep 10 & ps
echo ‘‘<<<<< Directory Operations ’’ cd /home/ugrads/ pwd cd − pwd
echo ‘‘<<<<< Miscellenous ’’ rm newpwd.txt jobs
sleep 10
echo ‘‘cat /proc/$(ps<<<<<|grep bashBonus ($−|headsign expansion)−1|awk ’{print $1}’)/status’’
• Continually prompt for textual user input on a command line.
• Parse user input according the provided grammar (see below)
• When a user enters a well formed command, execute it in the same way as a shell. You must use the commands fork and exec to accomplish this. You may NOT use the C++ system() command.
• Allow users to pipe the standard output from one command to the input of another an arbitrary number of times.
• Support input redirection from a file and output redirection to a file.
• Allow users to specify whether the process will run in the background or foreground using an ’&’. Backgrounding processes should not result in the creation of zombie processes. (Commands to run in the foreground do not have an ’&’, and commands that run in the background do)
• Print a custom prompt which supports printing the current directory, user name, current date, and current time.
• (Bonus) Allowing $-sign expansion. See the last sample command in the list of commands for example.
Write a report describing your design for piping, redirection and other special techniques that you are unique to your implementation. No need to restate the obvious.
Figure #4: Simple Shell Grammar
valid string = unix command || unix command AMP || special command
unix command = command name ARGS || unix command REDIRECTION filename ||
unix command PIPE unix command
special command = cd DIRECTORY || exit command name = any valid executable/interpreted file name
AMP = &
ARG = string
ARGS = ARG ARGS || ARG
DIRECTORY = absolute path || relative path
PIPE = |
REDIRECTION = < ||
Figure #4: Some Sample Commands
echo Here starts our shell ...
echo −e ‘‘<<<<< This message contains a line feed \n’’ echo ‘‘<<<<< Start to exercise pipe ’’
echo ‘‘<<<<< IO redirection ’’ ps test.txt
grep pts < test.txt pwd pwd.txt mv pwd.txt newpwd.txt cat newpwd.txt
echo ‘‘ 1 pipe ’’ ps
echo ‘‘ 2 pipes ’’
psps −−aa || awk ’/pts/[0awk ’/pts/[0−−9]9]//{{print $1print $2}}’’ || tailtail
echo ‘‘<<<<< 3 pipes ’’
lslsls −−−l /proc/sysl /proc/sysl /proc/sys ||| awk ’awk ’awk ’{{{print $9print $8$9print $8}}’’ }’|||headsorthead−−−r310|||headsortsort −−r5
echo ‘‘<<<<< 4 pipes with I/O redirection ’’
awk ’cat output.txtls −l /proc/sys{print $8$9}’ test.txt< test.txt | head −10 | head −8 | head −7 | sort output.txt
echo ‘‘<<<<< Background process (ps after some time to make sure the bg’s are not defunct) ’’
dd if=/dev/zero of=/dev/null bs=1024 count=10485760 &
sleep 10 & ps
echo ‘‘<<<<< Directory Operations ’’ cd /home/ugrads/ pwd cd − pwd
echo ‘‘<<<<< Miscellenous ’’ rm newpwd.txt jobs
sleep 10
echo ‘‘cat /proc/$(ps<<<<<|grep bashBonus ($−|headsign expansion)−1|awk ’{print $1}’)/status’’
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