Operating Systems and Systems Integration

Processes: Writing a Simple Shell 1 Aim

The successful student will write a very simple shell, i.e., a program that can interactively start other programs.

2

Background

A shell is a program that can start other programs. A real shell can do lots of other things (it supports a programming language, for example), but here we keep it very simple, and restrict it to starting other programs.

2.1

How do you Run an External Program as a New Process?

• Replace the instructions in a running process with a new set of instructions, using the exec function • First make an exact copy of your process using fork() • Then replace the contents of this new process with another program, using exec().

2.2

The exec*() functions

• There are six kinds of exec*() function; see man 3 exec and man 2 execve • We will use execl(): int execl(const char *path, const char *arg, ...); Parameter number: 1. gives full path of the program file you want to execute 2. gives name of the new process 3. specifies the command line arguments you pass to the program 4. (in this example) is a NULL pointer to end the paramter list. We must always put a NULL pointer at the end of this list. • Program 1 on the following page is a simple example, without error checking. As we saw in the lecture, Linux and Unix provide simple system calls to manage processes: #include <sys/types.h> #include <unistd.h> pid_t fork(void); Returns twice; returns 0 if child, returns child’s pid if parent, returns −1 if error.
Nick Urbanik <nicku@vtc.edu.hk> ver. 1.1

Processes: Writing a Simple Shell Operating Systems and Systems Integration

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Program 1 A simple program using fork() and execl(). It does no error checking.
#include <stdio.h> #include <unistd.h> int main() { printf ( "hello world\n" ); int pid = fork(); printf ( "fork returned %d\n", pid ); if ( pid == 0 ) execl( "/bin/ls", "ls", NULL ); else printf ( "I'm the parent\n" ); }

2.3

What Happens Between fork() and exec() and After?
◦ There is one process, the parent process.

• Before calling fork():

• After calling fork(): ◦ Two process are running, both still have the original code • After calling exec(): ◦ The child process, which called exec(), now has completely different code. Program 2 is called print.c and prints a number n times. Program 3 on the following Program 2 A simple program print.c that takes two numbers as parameters, and repeats the first number the number of times given by the second number.
#include <stdio.h> #include <stdlib.h> int main( int argc, char ∗argv [ ] ) { // argv[0] is the program name int num = atoi( argv [1] ); int loops = atoi( argv [2] ); int i; for ( i = 0; i < loops; ++i ) printf ( "%d ", num ); }

page is called call-print.c, and is written to call the program print.c, using execl().

Nick Urbanik <nicku@vtc.edu.hk>

ver. 1.1

Processes: Writing a Simple Shell Operating Systems and Systems Integration

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Program 3 A program call-print.c that uses execl() to call program 2 on the page before, print.c, in two separate processes.
#include <stdio.h> #include <unistd.h> int main() { printf ( "hello world\n" ); int pid = fork(); printf ( "fork returned %d\n", pid ); if ( pid == 0 ) execl( "./print", "print", "1", "100", NULL ); else execl( "./print", "print", "2", "100", NULL ); }

2.4

Exercise Set 1

1. Copy the programs from the network filesystem at /home/nfs/processes-and-threads to a new directory in your $HOME. 2. Compile and run program 1 on the preceding page. $ gcc -o fork-1 fork-1.c $ ./fork-1 3. Modify the program 1 on the page before so that it runs the program ls with the option -l. 4. Compile the program print.c in program 2 on the preceding page and run it: $ gcc -o print print.c $ ./print 10 5 Try running it with a few different numbers. 5. Compile the main program call-print.c in program 3 and run it. 6. Try printing each number: 100 times, 1000 times, 10,000 times, 100,000 times.

2.5

Implementing a Shell

Prompt user Get command If not time-to-exit Fork new process Replace new process with either who, ls or uptime Read next command Program 4 on the following page is a simple example shell.

Nick Urbanik <nicku@vtc.edu.hk>

ver. 1.1

Processes: Writing a Simple Shell Operating Systems and Systems Integration

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Program 4 A simple shell program, shell-1.c.
#include <stdio.h> #include <unistd.h> #include <stdlib.h> void print menu( void ) { printf ( "Enter 1=who, 2=ls, 3=uptime -> " ); } int main() { int cmd; print menu(); scanf ( "%d", &cmd ); while ( cmd != 0 ) { int pid = fork(); if ( pid == 0 ) { if ( cmd == 1 ) execl( "/usr/bin/who", "who", NULL ); if ( cmd == 2 ) execl( "/bin/ls", "ls", NULL ); if ( cmd == 3 ) execl( "/usr/bin/uptime", "uptime", NULL ); exit( 1 ); } /∗ add: wait( NULL ); here ∗/ print menu(); scanf ( "%d", &cmd ); } }

2.6

Exercise Set 2

1. Implement program 4 and run it. What if you give it a wrong number? 2. Open a second shell window, and monitor the creation of zombie processes by executing the command watch -n1 "ps aux | grep ’ [Z] ’" 3. Modify the program to print an error message if a command is not supported. 4. Add the the call to wait() in the loop before printing the menu. What is the difference in the behaviour of your program? 5. Modify the program and add two more commands to your shell, such as date and hostame. 6. Modify the program so that it will exit cleanly if it reads end of file. You can manu ¨ ally provide end of file to a process reading standard input by pressing  Control-d © . Hint: see man scanf. 7. Examine the program simplesh.c. Modify it to implement background or foreground processes.

Nick Urbanik <nicku@vtc.edu.hk>

ver. 1.1