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Everything you need to know to start coding your own shell

shell

Everything you need to know to start coding your own shell

Exercises

0. getppid Getting the process ID:
   Compile the code:

gcc -Wall -Werror -pedantic pid.c -o mypid && ./mypid

Run the program:

./mypid

1. /proc/sys/kernel/pid_max Get the max PID:
   Compile the code:

gcc -Wall -Werror -pedantic pid_max.c -o pid_max && ./pid_max

Run the code:

./pid_max

Sample output:

Maximum process ID: -1

Exercises 0. av

Write a program that prints all the arguments, without using ac.

1. Read line

Write a program that prints "$ ", wait for the user to enter a command, prints it on the next line.

man 3 getline

important make sure you read the man, and the RETURN VALUE section, in order to know when to stop reading.
Keyword: “end-of-file”, or EOF (or Ctrl+D).
Compile the code:

gcc -Wall -Wextra -Werror -pedantic prompt.c -o prompt

Run the program:

./prompt

Sample output:

./prompt 
$ cat prompt.c
cat prompt.c

2. command line to av

Write a function that splits a string and returns an array of each word of the string.

man strtok File: commandlinetoav Compile the code:

gcc -Wall -Wextra -Werror -pedantic commandlinetoav.c -o commandlinetoav

Run the program:

./commandlinetoav

Sample output:

shell# ./commandlinetoav
Number of words: 5
Words:
This
is
a
test
string

Executing a program

The system call execve allows a process to execute another program (man 2 execve). Note that this system
call does load the new program into the current process’ memory in place of the “previous” program: on
success execve does not return to continue the rest of the “previous” program.

Warning: in this example, execve is used without the current environment (last argument), don’t forget to add it in your Shell!
See exec.c

Compile the code:

gcc -Wall -Wextra -Werror -pedantic exec.c -o exec

Run the program:

./exec

Sample output:

Before execve
total 64
drwxr-xr-x 1 root root 20480 Mar 30  2022 bin
drwxr-xr-x 2 root root     6 Apr 15  2020 games
drwxr-xr-x 1 root root  4096 Mar 30  2022 include
drwxr-xr-x 1 root root  4096 Mar 30  2022 lib
drwxr-xr-x 1 root root  4096 Mar 30  2022 lib32
drwxr-xr-x 1 root root    34 Mar 30  2022 lib64
drwxr-xr-x 1 root root    95 Mar 30  2022 libexec
drwxr-xr-x 1 root root  4096 Mar 30  2022 libx32
drwxr-xr-x 1 root root    19 Mar 16  2022 local
drwxr-xr-x 1 root root  4096 Mar 30  2022 sbin
drwxr-xr-x 1 root root  4096 Mar 30  2022 share
drwxr-xr-x 2 root root     6 Apr 15  2020 src

Creating processes

The system call fork (man 2 fork) creates a new child process, almost identical to the parent (the process
that calls fork). Once fork successfully returns, two processes continue to run the same program, but with
different stacks, datas and heaps.
see fork.c.

Compile the code:

gcc -Wall -Wextra -Werror -pedantic fork.c -o fork

Run the program:

./fork

Sample output:

Before fork
After fork
My pid is 120
After fork
My pid is 121

Similarly, Using the return value of fork, it is possible to know if
the current process is the father or the child: fork will return 0 to the child,
and the PID of the child to the father.
See: fork2.c

Compile the code:

gcc -Wall -Wextra -Werror -pedantic fork2.c -o fork2

Run the program:

./fork2

Sample output:

shell# ./fork2
My pid is 192
(192) 193, I am your father
My pid is 193
(193) Nooooooooo!

Wait

The wait system call (man 2 wait) suspends execution of the calling process until one of its children terminates. See wait.c

Compile the code:

gcc -Wall -Wextra -Werror -pedantic wait.c -o wait

Run the program:

./wait

Sample output:

Wait for me, wait for me
Oh, it's all better now

Exercise: fork + wait + execve

Write a program that executes the command ls -l /tmp in 5 different child processes.
Each child should be created by the same process (the father).
Wait for a child to exit before creating a new child.

Compile the code:

gcc -Wall -Wextra -Werror -pedantic forkwaitexecve.c -o forkwaitexecve

Run the program:

./forkwaitexecve

Sample output:

shell# ./forkwaitexecve
Child process 1: PID 220
total 0
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.DAOEzJT0J6
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.dOKjikw0Ig
drwx------ 2 root  root  22 Mar 30  2022 tmpztrs6ymg
Child process 2: PID 221
total 0
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.DAOEzJT0J6
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.dOKjikw0Ig
drwx------ 2 root  root  22 Mar 30  2022 tmpztrs6ymg
Child process 3: PID 222
total 0
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.DAOEzJT0J6
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.dOKjikw0Ig
drwx------ 2 root  root  22 Mar 30  2022 tmpztrs6ymg
Child process 4: PID 223
total 0
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.DAOEzJT0J6
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.dOKjikw0Ig
drwx------ 2 root  root  22 Mar 30  2022 tmpztrs6ymg
Child process 5: PID 224
total 0
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.DAOEzJT0J6
drwx------ 2 mysql mysql  6 Mar 30  2022 tmp.dOKjikw0Ig
drwx------ 2 root  root  22 Mar 30  2022 tmpztrs6ymg

Exercise: super simple shell

gcc -Wall -Wextra -Werror -pedantic firstsimpleshell.c -o firstsimpleshell

Run the program:

./firstsimpleshell

Sample output:

shell# ./firstsimpleshell
#cisfun$ ls
av                 custom_getline  exec.c              fork     fork.c            getppid  pid.c      prompt     stat.c
commandlinetoav    env-main.c      firstsimpleshell    fork2    forkwaitexecve    main.h   pid_max    prompt.c   wait