Shell
Content
Backstory
Important Things to Note
Overview and To-Dos
Starting Your Shell
Interaction Within Your Shell
Built-in Commands
External Commands
Logical Operators
Memory
Background Processes
ps
Redirection Operators
Signal Commands
Grading
Learning Objectives
The learning objectives for Shell are:
Learning How a Shell Works
Fork, Exec, Wait
Signals
Processes
Zombie Processes
Backstory
Important Things to Note
Part 1 due 2024-09-23 23shell.c
Part 2 due 2024-09-30 23shell.c
Well, weʼll keep it short – you got fired from Macrohard. Your boss brought you in for a
code review and was more than disappointed. Apparently, they wanted a C++ style vector:
we didnʼt get the memo. Now, youʼve decided to work for insert hot tech company here,
and you got the job! However, thereʼs a catch - all newhires in insert hot tech company here
apparently have to go through a newcomers test if they want to keep their jobs. The task?
Write a shell. So, youʼre going to drop a shell that is so fancy that your boss will not
just keep you in the company, theyʼll immediately give you a pay raise as well.
The basic function of a shell is to accept commands as inputs and execute the
corresponding programs in response. You will be provided the vector , sstring and
format.h libraries for your use. Hopefully, this will make things right and you can secure
your foothold at insert hot tech company here. Feel free to refer to the Unix shell as a rough
reference.
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 1/14Fork Bombs
To prevent you from fork bombing your own VM, we recommend looking into 
ulimit
(https://man7.org/linux/manpage.
sT/h.i/sman3/ulimit.3p.html)
will allow you to set a limit for how many times you can fork. Note that 
ulimit
(https://man7.org/linux/manpage
iss/ t.e/rmmainn3a/l ulimit.3p.html)
session specific, so you will need to
do it everytime you launch a terminal
add this to your ~/.bashrc file (feel free to look up online how to do so), so that it is
run every time you log in to your VM.
Note that you should give it a more generous amount (say, 100-200), since the terminal will
likely have background processes already running. If you give it too small a limit, you wonʼt
be able to launch anything, and youʼll need to launch a new terminal.
If you happen to fork bomb your CS Cloud VM, please notify course staff in a private post
with your VM number. Note that it may take up to a few hours for us to respond, so try not
to fork bomb your VM.
Plan Before You Start
This assignment marks the beginning of a series of projects where you will be given mostly
blank files without predefined functions to fill in. Most of the remaining MPs will challenge
your design skills to create interesting utilities. Therefore, it is important that you read the
entirety of the documentation (including part 2), as well as the header files to get a
clear idea on what needs to be done. A few reminders about good coding and developing
practices that will really help you in the rest of the semester:
List down the features that you need to implement, as well as the gotchas. Make a todo
list to ensure you donʼt miss out anything.
Plan out the entirety of your assignment. Create a skeleton of how your entire code
will look like. This will prevent you from needing to restructure your entire code to add
in a single new feature.
Ensure that you fully understand the system calls/library functions youʼre using - the
parameters, the return values, the possible errors, the gotchas and notes.
Structure your code into modular functions. You do not want to debug a 1500 line
while loop within main .
Work incrementally. Implement a feature, test, debug, move on.
Good naming and spacing will make your code much more readable.
Try putting TODO comments in unfinished portions of your code. They are
automatically highlighted in many text editors, which alerts you to incomplete code.
Do Not Use 
system
(https://man7.org/linux/manpages/./man3/system.3p.html)
Since
a learning objective of this assignment is to use the fork-exec-wait pattern, if you use
system
(https://man7.org/linux/manpage,
sy/o.u/ wmailnl 3a/ustoymstaetimc.a3llpy. fhatilm tlh)is MP.
Input Formatting
Do not worry about irregular spacing in command inputs (i.e. extra whitespace before
and after each token). This is considered undefined behavior and will not be tested. You are
free to make your code as robust as you want, but we will only test the basic cases without
irregular spacing (unless specified).
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 2/14Overview and To-Dos
Starting Your Shell
Output Formatting
Since this MP requires your shell and the programs you launch to print a variety of things
like output messages and error messages, we have provided you with our own highly
customized formatting library. You should not be printing out to 
stdout
(https://man7.org/linux/manpage
asn/d./ man3/stdout.3p.html)
stderr
(https://man7.org/linux/manpage
ast/ .a/ll;man3/stderr.3p.html)
instead, all output and errors should be printed using the functions provided in format.h .
In format.h you can find documentation about what each function does, and you should
use them whenever appropriate.
If you place print statements in your debugging code, please remember to remove them
before autograding, or use the #define DEBUG block to place your print statements.
Note: donʼt worry if you donʼt use all of the functions in format.h , but you should use
them whenever their documented purpose matches the situation.
Flush Before Forking
Ensure that you 
fflush
(https://man7.org/linux/manpage
osu/.tp/umta fnil3e/ hfafnlduslehs. b3pe.fohrtem flo)rking. See
section 2.5.1 of the Open Group Base Specifications
(https://pubs.opengroup.o rfgo/ro mnloinrep inufbosr/m96at9io9n9 1o9n7 w99hy/f uthnicst iosns/V2_chap02.html#tag_15
necessary.
The shell is responsible for providing a command line for users to execute programs or
scripts. You should be very familiar with 
bash
(https://man7.org/linux/manpag
beys /n.o/wm,a wn1h/icbha swhil.l 1b.eh tthmel )basis for your own
shell. This is a 2 week MP, and the features you will need to implement are as follows:
Part 1
Starting up a shell
Optional arguments when launching shell
Interaction
Built-in commands
Foreground external commands
Logical operators
SIGINT handling
Exiting
Part 2
Everything from part 1, and:
Background external commands
ps
(https://man7.org/linux/manpages/./man1/ps.1p.html)
Redirection
commands
Signal commands
The shell should run in a loop like this executing multiple commands:
Print a command prompt
Read the command from standard input
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 3/14Print the PID of the process executing the command (with the exception of built-in
commands), and run the command
The shell must support the following two optional arguments, however, the order of the
arguments does not matter, and should not affect the functionality of your shell. Your shell
should be able to handle having none, one or both of these arguments.
History
Your shell should support storing the history of commands executed across shell sessions.
The command is as follows:
./shell -h <filename>
When provided -h , the shell should load in the history file as its history. Upon exiting, the
shell should append the commands of the current session into the supplied history file,
even if the shell is in a different working directory than where it started. If the file does not
exist, you should treat it as an empty file. The format of the history file stored should be
exactly the same as a script file, where you list a series of commands to be executed.
Example:
history.txt :
cd cs341
Hm
./shell -h history.txt
(pid=1234)/home/user/cs341$ echo Hey!
Command executed by pid=1235
Hey!
(pid=1234)/home/user/cs341$ exit
Updated history.txt :
cd cs341
Hm
echo Hey!
Notes:
If the the -h flag is not specified, the shell will still keep a history of commands run,
but will not read/write from/to a history file. Just think of it like private browsing mode
for your terminal.
Every command should be stored into the history file, unless specified.
File
Your shell should also support running a series of commands from a script file. The
command is as follows:
./shell -f <filename>
When provided -f , your shell will both print and run the commands in the file in sequential
order until the end of the file. See the following example file and execution:
commands.txt :
cd cs341
echo Hey!
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 4/14Interaction Within Your Shell
./shell -f commands.txt
(pid=1234)/home/user$ cd cs341
(pid=1234)/home/user/cs341$ echo Hey!
Command executed by pid=1235
Hey!
You have been given a sample script file test_file.txt . Your history files and script files
should be formatted in the same manner (this means you can use your history file as a
script file in -f ).
If the user supplies an incorrect number of arguments, or the script file cannot be found,
your shell should print the appropriate error from format.h and exit.
Tip: The 
getopt
(https://man7.org/linux/manpage
fsu/n.c/tmioann 3m/agye tcoopmte. 3inp .hhatnmdly). 
Prompting
When prompting for a command, the shell will print a prompt in the following format (from
format.h ):
(pid=<pid>)<path>$
<pid> is the process ID of the shell, and <path> is a path to the current working
directory. Note the lack of a newline at the end of this prompt.
Reading in Commands
The shell will read in a command from 
stdin
(https://man7.org/linux/manpag
 e(so/r .a/ fmilaen i3f /-sftd wina.s 3spp.ehctifmield)).
Command Types and Formats
Shell supports two types of commands: built-in and external (i.e. non-built-in). Built-in
commands are part of the shellʼs code, and are executed without creating a new process.
External commands must be executed by a new process, forked from your shell. If a
command is not one of the built-in commands listed, it is an external command.
Command arguments will be space-separated without trailing whitespace. Your shell does
not need to support quotes (for example, echo "hello there" ).
Running the Commands
The shell should run the command that was read in previously.
If the command is run by a new process, the PID of the process should be printed like this:
Command executed by pid=<pid>
This should be printed by the process that will run the command, before any of the output
of the command is printed (prints to be used are in format.h ).
Keeping History
Your shell should store the command that the user entered, so the user can repeat it later if
they wish. Every command should be stored unless otherwise noted. A vector may be
useful here.
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 5/14Built-in Commands
exit
(https://man7.org/linux/manpages/./man3/exit.3p.html)
The
shell will exit once it receives the 
exit
(https://man7.org/linux/manpag
ceosm/.m/maannd3 o/re oxnitc.e3 itp .rehctemilv)es an 
EOF
(https://man7.org/linux/manpagaets
t/h.e/man3/EOF.3const.html)
beginning of the line. An 
EOF
(https://man7.org/linux/manpa
 gise ss/e.n/t mbayn t3y/pEinOFg. 3Cctornls-tD. hfrtomml )your terminal. It is also sent
automatically from a script file (as used with the -f flag) once the end of the file is
reached. This should cause your shell to exit with exit status 0.
If there are currently stopped or running background processes when your shell receives
exit
(https://man7.org/linux/manpag
oers /C.o/nmtarno3l/-eDxi (tE.O3Fp)., hytomul s)hould kill and cleanup each of those children before your
shell exits. You do not need to worry about SIGTERM.
 If you donʼt handle 
EOF
(https://man7.org/linux/manpa
 goers /./man3/EOF.3const.html)
exit
(https://man7.org/linux/manpag
teos /e.xi/tm, ayon3u/ weixlli fta.il3 mp.ahntym olf) our test cases!
 Do not store 
exit
(https://man7.org/linux/manpag
iens h/i.s/tomrayn!3/exit.3p.html)
Catching Ctrl+C
Usually when we do Ctrl+C , the current running program will exit. However, we want the
shell itself to ignore the Ctrl+C signal ( SIGINT ) - instead, it should kill the currently
running foreground process (if one exists) using SIGINT . One way to do this is to use the
kill
(https://man7.org/linux/manpag
feusn/c.t/iomna no3n/ tkhiel lfo.r3epg.rhotumnld) process PID when SIGINT is caught in your shell.
However, when a signal is sent to a process, it is sent to all processes in its process group.
In this assignment, the shell process is the leader of a process group consisting of all
processes that are 
fork
(https://man7.org/linux/manpag‘de
 sf/ro.m/m iat.n S3o/f aonrokt.h3epr .whatym lto) properly handle Ctrl+C is to simply
do nothing inside the handler for SIGINT if it is caught in the shell - your shell will continue
running, but SIGINT will automatically propagate to the foreground process and kill it.
However, since we want this signal to be sent to only the foreground process, but not to
any backgrounded processes, you will want to use 
setpgid
(https://man7.org/linux/manpage
 st/o. a/smsaignn3 /esaecthp bgiadc.kg3pro.uhntdml)
process to its own process group after forking. (Note: think about who should be making
the 
setpgid
(https://man7.org/linux/manpage
 sc/a.ll/ amnadn 3w/hsye)t.pgid.3p.html)
There are several built-in commands your shell is expected to support.
cd <path>
Changes the current working directory of the shell to <path> . Paths not starting with /
should be followed relative to the current directory. If the directory does not exist, then
print the appropriate error. Unlike your regular shell, the <path> argument is mandatory
here. A missing path should be treated as a nonexistent directory.
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 6/14(pid=1234)/home/user$ cd code
(pid=1234)/home/user/code$ cd imaginary_directory
imaginary_directory: No such file or directory
(pid=1234)/home/user/code$
There is a system call that may be helpful here.
!history
Prints out each command in the history, in order.
(pid=1234)/home/user$ !history
0 ls -l
1 pwd
2 ps
(pid=1234)/home/user$
 This command is not stored in history.
#<n>
Prints and executes the -th command in history (in chronological order, from earliest to
most recent), where is a non-negative integer. Other values of will not be tested. The
command executed should be stored in the history. If is not a valid index, then print the
appropriate error and do not store anything in the history.
The following example assumes a fresh history:
(pid=1234)/home/user$ echo Echo This!
Command executed by pid=1235
Echo This!
(pid=1234)/home/user$ echo Another echo
Command executed by pid=1236
Another echo
(pid=1234)/home/user$ !history
0 echo Echo This!
1 echo Another echo
(pid=1234)/home/user$ #1
echo Another echo
Command executed by pid=1237
Another echo
(pid=1234)/home/user$ #**01
Invalid Index
(pid=1234)/home/user$ !history
0 echo Echo This!
1 echo Another echo
2 echo Another echo
(pid=1234)/home/user$
 Print out the command before executing if there is a match.
 The #<n> command itself is not stored in history, but the command being executed (if
any) is.
!<prefix>
Prints and executes the last command that has the specified prefix. If no match is found,
print the appropriate error and do not store anything in the history. The prefix may be
empty. The following example assumes a fresh history:
n
n n
n
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 7/14External Commands
(pid=1234)/home/user$ echo Echo This!
Command executed by pid=1235
Echo This!
(pid=1234)/home/user$ echo Another echo
Command executed by pid=1236
Another echo
(pid=1234)/home/user$ !e
echo Another echo
Command executed by pid=1237
Another echo
(pid=1234)/home/user$ !echo E
echo Echo This!
Command executed by pid=1238
Echo This!
(pid=1234)/home/user$ !d
No Match
(pid=1234)/home/user$ !
echo Echo This!
Command executed by pid=1239
Echo This!
(pid=1234)/home/user$ !history
0 echo Echo This!
1 echo Another echo
2 echo Another echo
3 echo Echo This!
4 echo Echo This!
(pid=1234)/home/user$
 Print out the command before executing if there is a match.
 The !<prefix> command itself is not stored in history, but the command being
executed (if any) is.
Invalid Built-in Commands
You should be printing appropriate errors in cases where built-in commands fail; for
example, if the user tries to 
cd
(https://man7.org/linux/manpa
ignetso/ a. /nmoanne1x/isctde.n1t pd.irhetcmtlo)ry.
(pid=1234)/home/user$ cd /imaginary_directory
/imaginary_directory: No such file or directory
(pid=1234)/home/user$
For commands that are not built-in, the shell should consider the command name to be the
name of a file that contains executable binary code. Such a code must be executed in a
process different from the one executing the shell. You must use 
fork
(https://man7.org/linux/manpag,
es/./man3/fork.3p.html)
exec
(https://man7.org/linux/manpag,
easn/d. /man3/exec.3p.html)
wait
(https://man7.org/linux
pag/es/./man3/wait.3p.ht
waitpid
(https://man7.org/linux/manpage.s/./man3/waitpid.3p.html)
The
fork/exec/wait paradigm is as follows: 
fork
(https://man7.org/linux/manpag
ae sc/h.il/dm parno3c/efsosr.k T.h3ep .chhtilmdl p)rocess
must execute the command with exec* , while the parent must 
wait
(https://man7.org/linux/manpag
feosr/ t.h/em cahni3ld/ wtaoit.3p.html)
terminate before printing the next prompt.
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 8/14Logical Operators
You are responsible of cleaning up all the child processes upon termination of your
program. It is important to note that, upon a successful execution of the command, 
exec
(https://man7.org/linux
pages/./man3/exec.3p.ht
never returns to the child process. 
exec
(https://man7.org/linux/manpag
oens/ly. r/emtaunrn3s/ etoxe tch.e3 cph.ihldt mplro)cess when the
command fails to execute successfully. If any of 
fork
(https://man7.org/linux/manpag,
es/./man3/fork.3p.html)
exec
(https://man7.org/linux/manpag,
eosr/ ./man3/exec.3p.html)
wait
(https://man7.org/linux/manpag
feasil/, .th/mea anp3p/rwoapirtia.t3ep.html)
error messages should be printed. The child should 
exit
(https://man7.org/linux/manpag
wesit/h. e/xmiat ns3ta/teuxsi 1t .if3 ipt .fahitlsm lto)
execute a command.
Some external commands you may test to see whether your shell works are:
/bin/ls
echo hello
Tip: It is good practice to flush the standard output stream before the fork to be able to
correctly display the output. This will also prevent duplicate printing from the child process.
 Please read the disclaimer at the top of the page! We donʼt want to have to give any
failing grades. 
Like 
bash
(https://man7.org/linux/manpag,
eyso/u.r/ smhaenl1l s/hboaushld. 1su.hptpmolrt) && , || , and ; in between two commands. This will
require only a minimal amount of string parsing that you have to do yourself.
Important: each input can have at most one of && , || , or ; . You do not have to support
chaining (e.g. x && y || z; w ).
Important: you should not try to handle the combination of the !history , #<n> , !
<prefix> , or 
exit
(https://man7.org/linux/manpag
ceosm/.m/maannd3s/ wexitiht a.n3yp .lohgtimcla)l operators. Rather, you can assume these
commands will always be run on a line by themselves.
Important: logical operators are stored in the history as one entry. This means your history
should look like so
(pid=1234)/home/user$ echo foo && echo bar
Command executed by pid=1235
foo
Command executed by pid=1236
bar
(pid=1234)/home/user$ !history
0 echo foo && echo bar
AND
&& is the AND operator. Usage:
x && y
The shell first runs x , then checks the exit status.
If x exited successfully (status = 0), run y .
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 9/14If x did not exit successfully (status ≠ 0), do not run y . This is also known as
short-circuiting
(https://en.wikipedia.org/wiki/Shortcircuit_e.valuation)
(pid=27853)/home/user/semester/shell$
echo hi && echo bye
Command executed by pid=27854
hi
Command executed by pid=27855
bye
(pid=27879)/home/mkrzys2/fa19/shell$ cd /asdf && echo short-circuit
/asdf: No such file or directory!
This mimics short-circuiting AND in boolean algebra: if x is false, we know the result will
be false without having to run y .
 This is often used to run multiple commands in a sequence and stop early if one fails.
For example, make && ./shell will run your shell only if 
make
(https://man7.org/linux/manpag
seusc/c.e/emdans.1/make.1p.html)
Tip: You may want to look into the provided macros to read the status of an exited child.
OR
|| is the OR operator. Usage:
x || y
The shell first runs x , then checks the exit status.
If x exited successfully, the shell does not run y . This is short-circuiting.
If x did not exit successfully, run y .
(pid=27853)/home/user/semester/shell$ echo hi || echo bye
Command executed by pid=27854
hi
(pid=27853)/home/user$ cd /asdf || echo runMe
/asdf: No such file or directory
Command executed by pid=27854
runMe
Boolean algebra: if x is true, we can return true right away without having to run y .
 This is often used to recover after errors. For example, make || echo 'Make
failed!' will run 
echo
(https://man7.org/linux/manpag
oens/ly. i/f man3/echo.3x.html)
make
(https://man7.org/linux/manpag
deso/e.s/ nmoatn s1u/cmcaekeed..1p.html)
Separator
; is the command separator. Usage:
x; y
The shell first runs x .
The shell then runs y .
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 10/14Memory
Background Processes
ps
(https://man7.org/linux/manpages/./man1/ps.1p.html)
(pid=27879)/home/user/semester/shell$
echo hi; echo bye
Command executed by pid=27883
hi
Command executed by pid=27884
bye
(pid=27879)/home/user/semester/shell$ cd /asdf; echo runMe
/asdf: No such file or directory
Command executed by pid=27884
runMe
 The two commands are run regardless of whether the first one succeeds.
As usual, you may not have any memory leaks or errors. Note that still reachable memory
blocks do not count as memory leaks.
An external command suffixed with & should be run in the background. In other words, the
shell should be ready to take the next command before the given command has finished
running. There is no limit on the number of background processes you can have running at
one time (aside from any limits set by the system).
There will be a single space between the rest of the command and & . For example, pwd &
is valid while you need not worry about pwd& .
Since spawning a background process introduces a race condition, it is okay if the prompt
gets misaligned as in the following example:
(pid=18**)/home/user$ pwd & 
Command executed by pid=1874 
(pid=18**)/home/user$ 
/home/user 
When I type, it shows up on this line 
Note this is not the only way your shell may misalign.
While the shell should be usable after calling the command, after the process finishes, the
parent is still responsible for waiting on the child. Avoid creating zombies! Do not catch
SIGCHLD , as catching SIGCHLD comes with all sorts of caveats and subtleties that are
hard to work around. Instead regularly check to see if your children need reaping (think
about placement of this piece of code: where should you put this, and why). Think about
what happens when multiple children finish around the same time, and what happens if a
foreground/background process finish around the same time.
Backgrounding will not be chained with the logical operators nor with redirection operators.
Like our good old 
ps
(https://man7.org/linux/manpa,
gyeosu/r. s/hmealnl s1h/opus.ld1 pp.rihntt molu)t information about all currently executing
processes. You should include the shell and its immediate children, but donʼt worry about
grandchildren or other processes. Make sure you use print_process_info_header() ,
print_process_info() , and time_struct_to_string() (and maybe some other helper
functions)!
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 11/14Redirection Operators
Note: while 
ps
(https://man7.org/linux/manpa
igse nso/r.m/maallyn 1a/ spesp.a1rpa.thet bmiln)ary, it is a built-in command for your shell. (This is
not “execing 
ps
(https://man7.org/linux/manpa”,g
tehsi/s .is/ myaonu1 i/mppsl.e1mpe.nhttinmlg) it in the code. Thus you may have to keep track
of some information for each process.)
Your version of the 
ps
(https://man7.org/linux/manpa
sgheosu/.ld/ mparinn1t /thpse. f1opll.ohwtinmgl) information for each process:
PID: The pid of the process
NLWP: The number of threads currently being used in the process
VSZ: The program size (virtual memory size) of the process, in kilobytes (1 kilobyte =
1024 bytes)
STAT: The state of the process
START: The start time of the process. You will want to add the boot time of the
computer (
btime
(htt)p,s a:/n/ldin sutxa.rdti eti.mneet /omf athne/5 p/prorocce)ss (
starttime
(https):/ /tloin cuaxl.cduiela.ntee tt/hmisa.n/5/proc)
Make sure you are careful while converting from various formats - the man pages for
procfs
(http hsa:/v/lein huex.lpdfieu.ln teipt/sm. an/5/proc)
TIME: The amount of cpu time that the process has been executed for. This includes
time the process has been scheduled in user mode (
utime
(https://man7.org/linux/manpag)e
 sa/n.d/ kmearnn3e/l umtoidmee. (3p.html)
stime
(https://man7.org/linux/manpag)e.s/./man2/stime.2.html)
COMMAND:
The command that executed the process
Some things to keep in mind:
The order in which you print the processes does not matter.
The ‘commandʼ for print_process_info should be the full command you executed.
The & for background processes is optional. For the main shell process only, you do
not need to include the command-line flags. Ensure that the ‘commandʼ does not
have trailing whitespace at the end of it.
You may not exec the 
ps
(https://man7.org/linux/manpa
bgeinsa/r.y/ tmoa cno1m/ppsle.t1ep .thhitsm pla)rt of the assignment.
Example output of this command:
(pid=25497)/home/user$ ps
PID NLWP VSZ STAT START TIME COMMAND
25498 1 7**8 R 14:03 0:08 dd if=/dev/zero bs=1M count=123456 
of=/dev/null &
25501 1 7288 S 14:04 0:00 sleep 1000 &
25497 1 7484 R 14:03 0:00 ./shell
Hint: You may find the /proc filesystem to be useful, as well as the man pages for it.
Your boss wants some way for your shell commands to be able to link together. You decide
to implement >> , > , and < . This will require only a minimal amount of string parsing that
you have to do yourself.
Important: each input can have at most one of >> , > or < . You do not have to support
chaining (e.g. x >> y < z > w ).
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 12/14Important: you should not try to handle the combination of the 
cd
(https://man7.org/linux/manpa,
g!ehsi/s.t/omrayn1, /#c<dn.>1p, .!html)
<prefix> , 
ps
(https://man7.org/linux/manpa,
goers /./man1/ps.1p.html)
exit
(https://man7.org/linux/manpag
ceosm/.m/maannd3s/ wexitiht a.n3yp .rehdtimrelc)tion operators. Rather, you can
assume these commands will always be run on a line by themselves.
Note: Assume that the redirection operator commands will be formatted correctly. Any
incorrectly formatted redirection commands is considered undefined behavior.
OUTPUT
> places the output of a command into a file. Usage:
<cmd> [args ...] > <filename>
If the file exists, overwrite the contents of the file with the output of the current command.
Example usage:
(pid=2777)/home/usr$ echo hello > hey.txt
Command executed by pid=3750
(pid=2777)/home/usr$ cat hey.txt
Command executed by pid=3751
hello
(pid=2777)/home/usr$ echo welcome to cs341 > hey.txt
Command executed by pid=3752
(pid=2777)/home/usr$ cat hey.txt
Command executed by pid=3754
welcome to cs341
APPEND
>> appends the output of a command into a file. Usage:
<cmd> [args ...] >> <filename>
If the file does not exist, assume that it is an empty file. Example usage ( hi.txt does not
exist in the directory before these commands are executed):
(pid=2777)/home/usr$ echo a >> hi.txt
Command executed by pid=2780
(pid=2777)/home/usr$ cat hi.txt
Command executed by pid=2781
a
(pid=2777)/home/usr$ echo wheeee >> hi.txt
Command executed by pid=2782
(pid=2777)/home/usr$ cat hi.txt
Command executed by pid=2783
a
wheeee
INPUT
< pipes the contents of a file into a command as its input. Usage:
<cmd> [args ...] < <filename>
If the file does not exist, it is undefined behavior. Example usage: hello.txt contains:
welcome to cs341
2024/9/26 21:42 CS 341 · Shell
https://cs341.cs.illinois.edu/assignments/shell#background-processes 13/14Signal Commands
Grading
(pid=3771)/home/usr$ wc < hello.txt
Command executed by pid=3772
 1 3 17
Hint: 
dup
(https://man7.org/linux/manpa
 gweisll/ b.e/ muasne3fu/ld fuopr. a3llp t.hhet mreld)irection commands
Like bash, your shell will support sending signals to its child processes. We require you to
implement the 3 signals listed below.
kill <pid>
The ever-useful panic button. Sends SIGKILL to the specified process.
Use the appropriate prints from format.h for:
Successfully sending SIGKILL to process
No process with pid exists
kill
(https://man7.org/linux/manpag
wesa/s. r/amna wn3it/hkoiultl a. 3ppi.dhtml)
stop <pid>
This command will allow your shell to stop a currently executing process by sending it the
SIGSTOP signal. It may be resumed by using the command cont .
Use the appropriate prints from format.h for:
Process was successfully sent SIGSTOP
No process with pid exists
stop was ran without a pid
cont <pid>
This command resumes the specified process by sending it SIGCONT .
Use the appropriate prints from format.h for:
Process was successfully sent SIGCONT
No such process exists
cont was ran without a pid
Note: Any <pid> used in 
kill
(https://man7.org/linux/manpag,
est/o.p/m, aonr,3 /ckoinltl .w3ipll. ehitthmelr) be a process that is a direct
child of your shell or a non-existent process. You do not have to worry about killing other
processes.
Note that Week 1 and Week 2 count as one week of MP grades respectively. See the
overview for a list of features required for each week.
2024/9/26 21:42 CS 341 · Shell