create your own shell by adding all the features described in the BLS lecture

INSTRUCTIONS TO CANDIDATES

ANSWER ALL QUESTIONS

100 Points

Summary

Use the code in CSAPP2, 8.4.6 "Using fork and execve to Run Programs" as a starting point to develop and test a small version of a shell that forks processes in the foreground and/or the background.

What To Do

Follow the directions in BLS "How the Shell Works" lecture.

Start with program shellex.c from figures 8.22-8.24 and create your own shell by adding all the features described in the BLS lecture: read and parse command line, run command in foreground or background, implement redirection, implement pipe.

Notes:

Note 1: Although a real shell can accept a command line with an arbitrary number of pipes, in this homework it is sufficient to implement just one pipe. So, your shell should be able to handle correctly a command line like

command-1 | command-2 < input-file > output-file &

Notice that the placement of "< input-file" and "> output-file" in the command line is not critical since "< input-file" redirects the input of command-1 and "> output-file" redirects the output of command-2. If there is no pipe, "> output-file" redirects the output of command-1.

Note 2: Your shell program can execute programs in the current directory but does not inherit PATH automatically, so

> ls

does not work, but

> /usr/bin/ls works

and, this works (when myspin is in the current directory)

> myspin 10 &

 

Note 3: Files csapp.c, csapp.h, shellex.c, myspin.c are in FILES/PROGRAMS To compile and run shellex.c do the following

gcc -c csapp.c

gcc -o shellex shellex.c csapp.o -lpthread

 

 

 

/* $begin shellmain */ #include "csapp.h"

 

#define MAXARGS 128

 

/* function prototypes */ void eval(char*cmdline);

int parseline(char *buf, char **argv); int builtin_command(char **argv);

 

int main()

{

char cmdline[MAXLINE]; /* command line */

 

while (1) {

/* read */ printf("> ");

Fgets(cmdline, MAXLINE, stdin); if (feof(stdin))

exit(0);

 

/* evaluate */ eval(cmdline);

}

}

/* $end shellmain */

 

/* $begin eval */

/* eval - evaluate a command line */ void eval(char *cmdline)

{

char *argv[MAXARGS]; /* argv for execve() */

char buf[MAXLINE]; /* holds modified command line */ int bg; /* should the job run in bg or fg? */

pid_t pid; /* process id */

 

strcpy(buf, cmdline);

bg = parseline(buf, argv); if (argv[0] == NULL)

return; /* ignore empty lines */

 

if (!builtin_command(argv)) {

if ((pid = Fork()) == 0) { /* child runs user job */ if (execve(argv[0], argv, environ) < 0) {

printf("%s: Command not found.\n", argv[0]); exit(0);

}

}

 

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