Description
In this homework, you will build something similar to a Linux shell. Your
shell, however, will not get commands iteratively from the user, but it will
parse some files to determine which commands to run.
Please, ask general questions about the homework on ICON, so that everyone
can benefit from the answers.
General Requirements Submit your homework as a single tar file.
When unpacked, the tar file must have the following directory structure
(substitute hyour HawkIDi with your actual HawkID):
hyour HawkIDi
compile.sh
printargs.c
printargsandenv.c
shell.c
The .sh file must have the executable bit set. No other files should be
present in the archive.
You must write your code using the language C.
Preliminary Tasks
1. Write a C program printing to sdout all its arguments (including
argv[0]), one per line. This is similar to what you had to write for
Homework 1 – task8. However, this time you do not have to hex-encode
the printed arguments. Save the source code of this program in printargs.c.
2. Write a C program printing to sdout all its arguments (including argv[0]),
one per line, followed by all its environment variables (again, one per line).
You can access environment variables using the following variant for your
main function:
int main(int argc, char *argv[], char* env[])
The environment variables are stored as an array of strings in the variable
env. This array is terminated by an element whose value is NULL. Save the
source code of this program in printargsandenv.c.
1
If you run the code of this C program in the following way:
./printargsandenv argument1 argument2
the output should be similar to:
./printargsandenv
argument1
argument2
XDG VTNR=7
ORBIT SOCKETDIR=/tmp/orbit-antoniob
XDG SESSION ID=c1
VIRTUALENVWRAPPER SCRIPT=/usr/share/virtualenvwrapper/virtualenvwrapper.sh
. . .
XAUTHORITY=/home/antoniob/.Xauthority
COLORTERM=gnome-terminal
=./printargsandenv
OLDPWD=/home/antoniob/git
(I omitted many lines. Also, the values of your environment variables will
likely be different.)
3. Copy the file shell.c I provided in the homework tar file. This file
contains some of the code you will need to use to write your shell.
4. Write a shell script named compile.sh. This script must compile
the code in printargs.c in an program called printargs, the code in
printargsandenv.c in an program called printargsandenv, and the code
in shell.c in an program called shell.
The printargs and printargsandenv programs will be useful to test the
functionality of your shell. The code I provided in shell.c constitutes a
starting point you should use to write your shell.
In the following sections, I will also refer to some .txt files. You can find
them in the tar file of the homework. When testing your shell, I suggest to
place them in the same folder where your shell compiled program is. This
folder should also contain printargs and printargsandenv.
You can assume that your shell will be run using hyour HawkIDi as the
current working directory.
2
Main Task
You need to write a C program (save its source code in the file called shell.c)
similar to a Linux shell. Your shell, however, will not get commands iteratively from the user. Conversely, it will parse some files to determine which
commands to run.
In particular, your shell must accept as arguments an arbitrary amount
of paths to command files. A command file is a text file containing, one
per line, the following fields:
hbinary pathi
hstdini
hstdouti
hstderri
hargumentsi
hextra environmenti
huse pathi
hcopy environmenti
hnicenessi
hwaiti
htimeouti
The meaning of each field will be explained below.
In general, every command file specifies a subcommand your shell should
execute and some properties regarding how it should be executed. The commands described in the command files specified as arguments of your shell
should be executed in order.
The file shell.c, which you can find in the homework tar file, provides a
scaffolding for the code of your shell. In particular, it provides code to parse
a command file and generate a variable of type command. You can assume
that if a command file is correctly parsed by the function parse command
(i.e., the function parse command returns 1), all the parsed values respect
the specification given below.
All the commands must be executed in different child processes of the
main parent process. Every time a new process is created, your shell must
print (in a separate line):
“New child process started hnew process pidi”
Every time a child process terminates, your shell must print (in a separate
line):
“Child process hchild process pidi terminated with exit code
3
hchild process exit codei”
Your shell must not terminate before all the executed commands are
terminated as well.
The aforementioned fields of a command file have the following meaning:
hbinary pathi: This is the path (absolute or relative) of the program you
need to execute.
hstdini: If this field is not an empty string, the command’s stdin should
be read from the specified file, instead than from the terminal.
hstdouti: If this field is not an empty string, the command’s stdout
should be redirected to the specified file.
hstderri: If this field is not an empty string, the command’s stderr should
be redirected to the specified file.
hargumentsi: Specify the arguments of the program you have to run.
The arguments are hex-encoded, you have to write your own code to
hex-decode them.
When hex-encoded, the different arguments are separated by the string
“00”, therefore, once decoded, the different arguments will be separated by
a NULL byte.
hextra environmenti: Specify some environment variables you should
set for the executed program. This field is hex-encoded. As for hargumentsi,
when hex-encoded, the different extra environment variables are separated
by the string “00”, therefore, once decoded, the different arguments will be
separated by a NULL byte.
huse pathi: This value can be either 0 or 1. If 1, your shell should search
for hbinary pathi in all the directories specified by the PATH environment
variable. You can obtain this behavior by using the execvpe function call.
hcopy environmenti: This value can be either 0 or 1. If 0, the environment variables of the shell should not be propagated to the executed command. If 1, the environment variables of the shell should be propagated to
the executed command. In both cases, the environment variables potentially
specified in hextra environmenti, should be set in the executed command.
hnicenessi: This value is a number between -20 and 19. You need to set
the niceness of the executed command to this value (use the setpriority
function in the child process, after fork, but before execv).
hwaiti: This value can be either 0 or 1. If 0, the shell will not wait for
the termination of this command, before the execution of the next command.
Otherwise, the shell will wait for the termination of the execution of this
command, before executing the next one (if present). Please notice that,
4
regardless of the value of hwaiti, before exiting, the shell has to wait for the
termination of all the launched commands. As soon as any child process
is terminated, the shell has to print out the pid of the terminated process
together with its exit code, as specified before.
htimeouti: This value is an integer number greater or equal than 0.
If different than 0, you have to terminate the executed command after
htimeouti seconds. To implement this functionality, use the utility called
timeout. In particular, suppose that the command you need to execute
is ls -la and htimeouti is set to 15. Then, instead of just executing the
command ls -la, you should execute the following command:
/usr/bin/timeout –preserve-status -k 1 15 ls -la
Other Details and Hints
When calling execv and any of its variants, the valued specified as path and
the value specified as argv[0] must always be the same. This must be true
even when running a command with a timeout. The different arguments
eventually specified by the field hargumentsi in the command file, indicates
what you should pass as argv[1], argv[2], …
To use the function execvpe, you need to add the following line at the
beginning of your source file:
#define GNU SOURCE
In addition, execvpe does not work if the PATH environmental variable is not
set. This is a problem when the command does not inherit this environment
variable from the parent process (i.e., hcopy environmenti is set to 0). You
can ignore this issue.
Setting a niceness value lower than 0 does not work unless the shell is
executed as the root user. You can ignore this issue.
You should be carefully in how you open the files used for stdin, stdout,
and stderr redirection. First of all, you can assume that, if specified, the
stdin file exists. Likewise, you can assume that the directories containing
stdout and stderr already exist and they are writable by your shell’s user.
If already existing, you should delete the content of the stdout and stderr
files, before writing into them. You can get the aforementioned behaviors by
using:
open(path, O WRONLY|O CREAT|O TRUNC, 0664)
to open the files used as stdout and stderr, and using:
5
open(path, O RDONLY, 0664)
for the file used as stdin.
Examples
You can find the files mentioned in this section in the tar file of the homework. Also, these examples assume that you have the program printargs
and printargsandenv already compiled in the same directory where you
compiled your shell. Obviously, some output, such as the values of the
different pids or file timestamps, may be different.
By running:
./shell ls.txt
Your output should be:
New child process started 31007
total 180
drwxr-xr-x 12 root root 4096 Aug 30 16:39 .
drwxr-xr-x 26 root root 4096 Aug 30 16:39 ..
drwxr-xr-x 2 root root 102400 Sep 4 16:26 bin
drwxr-xr-x 2 root root 4096 Aug 30 16:42 games
drwxr-xr-x 76 root root 4096 Aug 30 16:44 include
drwxr-xr-x 174 root root 16384 Sep 4 16:24 lib
drwxr-xr-x 3 root root 4096 Aug 30 16:40 lib32
drwxr-xr-x 3 root root 4096 Aug 30 16:40 libx32
drwxr-xr-x 10 root root 4096 Jul 24 22:03 local
drwxr-xr-x 2 root root 12288 Aug 30 16:59 sbin
drwxr-xr-x 428 root root 16384 Sep 4 15:07 share
drwxr-xr-x 7 root root 4096 Aug 30 14:04 src
Child process 31007 terminated with exit code 0
By running:
./shell ls2.txt
Your output should be:
New child process started 31340
ls: cannot access ‘ZZZnonexistingfile’: No such file or directory
Child process 31340 terminated with exit code 2
6
By running:
./shell sleeptimeout.txt
Your output should be:
New child process started 29172
Child process 29172 terminated with exit code 143
The last line should be printed approximately 10 seconds after the first one.
By running:
./shell printargsenv.txt
Your output should be:
New child process started 29317
./printargsandenv
100
ZZZ=z
ZZZ=z
YZZ=z
Child process 29317 terminated with exit code 0
By running:
./shell sleeptimeout.txt bash.txt stracegrep.txt
Your output should be:
New child process started 29593
New child process started 29594
Child process 29594 terminated with exit code 0
New child process started 29596
Child process 29596 terminated with exit code 0
Child process 29593 terminated with exit code 143
The last line should be printed approximately 10 seconds after the others.
Your shell should also create a file called stdout, containing the word hello,
and a file called stderr containing the list of all the system calls executed by
the command grep (obtained by using strace). Both files should be saved
in the shell’s current working directory.
7
By running:
./shell sleep.txt sleeplonger.txt
Your output should be:
New child process started 4610
New child process started 4611
Child process 4610 terminated with exit code 0
Child process 4611 terminated with exit code 0
The third line of your shell’s output should be printed approximately 5 seconds after the start of your shell. The fourth line should be printed approximately 10 seconds after the start of your shell. The third line should contain
the pid of the first started child process and the fourth line should contain
the pid of the second started child process.
8