digenv.c

/*
 *
 * NAME:
 *   digenv  -  a program for easy handling of environmental variables
 *
 * SYNTAX:
 *   ./digenv [options] PATTERN [FILE...]
 *   ./digenv [options] [-e PATTERN | -f FILE] [FILE...]
 *
 * DESCRIPTION:
 *   The program digenv is supposed to help the user display the
 *   environment variables of the system in an easy way using
 *   pipes and built in system calls.
 *
 * OPTIONS:
 *   see grep(1)
 *
 * EXAMPLES:
 *   ./digenv -i PATH
 *
 * SEE ALSO:
 *   grep(1), sort(1), printenv(1), less(1), more(1)
 *
 * NOTES:
 *   If any parameter is entered in the wrong way, the program will exit.
 *
 * AUTHOR:
 *   Rickard Eriksson   -   rieri@kth.se
 *   Rebecca Ahlstrand  -   reah@kth.se
 */


#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>

#define READ_PIPE (0)   /* defines that the integer 0 is bound to READ_PIPE */
#define WRITE_PIPE (1)  /* defines that the integer 1 is bound to WRITE_PIPE */

pid_t printenvpid;  /* holds the pid for the child process printenv */
pid_t greppid;      /* holds the pid for the child process grep */
pid_t sortpid;      /* holds the pid for the child process sort */
pid_t pagerpid;     /* holds the pid for the child process pager */

int pipe_print[2];  /* variable to hold the file descriptors for the function printenv */
int pipe_grep[2];   /* variable to hold the file descriptors for the function grep */
int pipe_sort[2];   /* variable to hold the file descriptors for the function sort */
int ret;            /* variable to hold returns from system calls for error handling */

/* Function pipe_close(int pipe[])
 *
 * The function takes an int array that contains the file descriptors of a pipe and
 * closes them. There's also error handling if any of the system calls gets an error.
 */

void pipe_close(int p[]) {
    ret = close(p[READ_PIPE]);      /* system call to close to end the READ end of a pipe */
    if (ret < 0) {                  /* if ret < 0 then there has been an error and the program exits */
        perror("Read pipe closure failed");
        exit(EXIT_FAILURE);
    }

    ret = close(p[WRITE_PIPE]);     /* system call to close to end the WRITE end of a pipe */
    if (ret < 0) {                  /* if ret < 0 then there has been an error and the program exits */
        perror("Write pipe closure failed");
        exit(EXIT_FAILURE);
    }
}

/* Function printenv()
 *
 * Writes all the environment variables on the system to the pipe pipe_print. If errors occur
 * there is error handling.
 */

void printenv() {

    char *args[] = { "printenv", NULL };        /* array that defines which system call to use */

    ret = dup2(pipe_print[WRITE_PIPE], STDOUT_FILENO);      /* dup2 copys the file descriptor from stdout to the pipe */
    if (ret < 0) {              /* if ret < 0 then there has been an error and the program exits */
        perror("Dup failed");
        exit(EXIT_FAILURE);
    }

    pipe_close(pipe_print);     /* closing of pipe after use */

    execvp(args[0], args);      /* executes the program defined in args[0] */

    perror("exec failed");
    exit(EXIT_FAILURE);
}

/* Function grep(char **arguments)
 *
 * Takes a list of parameters to pass on to the system function grep(1) and uses these on the list passed on from printenv
 * via the pipe pipe_print.
 */

void grep(char **arguments) {

    ret = dup2(pipe_print[READ_PIPE], STDIN_FILENO);        /* dup2 copys the file descriptor from stdin to the pipe */
    if (ret < 0) {              /* if ret < 0 then there has been an error and the program exits */
        perror("Dup failed");
        exit(EXIT_FAILURE);
    }

    /* set write end of pipe as stdout for this child process */
    ret = dup2(pipe_grep[WRITE_PIPE], STDOUT_FILENO);       /* dup2 copys the file descriptor from stdout to the pipe */
    if (ret < 0) {              /* if ret < 0 then there has been an error and the program exits */
        perror("Dup failed");
        exit(EXIT_FAILURE);
    }

    pipe_close(pipe_print);     /* closing of pipe after use */
    pipe_close(pipe_grep);      /* closing of pipe after use */

    execvp("grep", arguments);  /* executes the program defined in args[0] */

    perror("exec failed");
    exit(EXIT_FAILURE);
}

/* Function sort(int p[])
 *
 * Sorts a list of environment variables in alphabetical order. The list of variables are
 * either all the variables passed from printenv or the function grep depending on the users
 * choice. int p[] is the pipe that contains the list that is to be sorted.
 */

void sort(int p[]) {
    char *args[] = { "sort", NULL };            /* array that defines which system call to use */

    ret = dup2(p[READ_PIPE], STDIN_FILENO);     /* dup2 copys the file descriptor from stdin to the pipe */
    if (ret < 0) {              /* if ret < 0 then there has been an error and the program exits */
        perror("Dup failed");
        exit(EXIT_FAILURE);
    }

    ret = dup2(pipe_sort[WRITE_PIPE], STDOUT_FILENO);       /* dup2 copys the file descriptor from stdout to the pipe */
    if (ret < 0) {              /* if ret < 0 then there has been an error and the program exits */
        perror("Dup failed");
        exit(EXIT_FAILURE);
    }

    pipe_close(p);              /* closing of pipe after use */
    pipe_close(pipe_sort);      /* closing of pipe after use */

    execvp(args[0], args);      /* executes the program defined in args[0] */

    perror("exec failed");
    exit(EXIT_FAILURE);
}

/* Function pager()
 *
 * The function that decides the pager that is to be used when displaying the sorted list.
 * If the environment variable "PAGER" contains a default the program will use it,
 * otherwise the default is to use less().
 */

void pager() {

    char *args[] = { getenv("PAGER"), NULL };

    if (args[0] == NULL) {
        args[0] = "less";
    }

    /* set write end of pipe as stdout for this child process */
    ret = dup2(pipe_sort[READ_PIPE], STDIN_FILENO);         /* dup2 copys the file descriptor from stdin to the pipe */
    if (ret < 0) {              /* if ret < 0 then there has been an error and the program exits */
        perror("Dup failed");
        exit(EXIT_FAILURE);
    }

    pipe_close(pipe_sort);      /* closing of pipe after use */

    ret = execvp(args[0], args);    /* executes the program defined in args[0] */
    if (ret < 0) {                  /* if ret < 0 then there has been an error */
        args[0] = "more";           /* if the system can't find less(), then more() is used */
        execvp(args[0], args);      /* executes the program defined in args[0] */
    }

    perror("exec failed");
    exit(EXIT_FAILURE);
}

/* The main program
 *
 * The program takes two parameters, first the input counter and then the array och inputs.
 * Main.c is the program that creates all the child processes, pipes and handles parts of
 * the error handling. At the end of the program it waits for all the child-processes to
 * finish.
 */

int main(int argc, char **argv) {

    int status;        /* Variable to indicate the status of a child process */

    if (pipe(pipe_print) < 0) {         /* opening pipe to printenv, if the returned value < 0 an error has occured */
        perror("pipe failed");
        return EXIT_FAILURE;
    }

    printenvpid = fork();               /* creates a child process for the printenv function */
    if (printenvpid < 0) {              /* if this is TRUE, an error has occured and the program is shut down */
        perror("fork failed");
        return EXIT_FAILURE;
    }

    if (printenvpid == 0) {             /* if this is TRUE then the program is in the child process */

        printenv();                     /* a call to the function printenv() */

    }

    if (argc > 1) {                     /* if this is TRUE, input to the function grep exists. If FALSE then the
                                         * program skips the grep() function and goes directly to sort()
                                         */
        if (pipe(pipe_grep) < 0) {      /* opening pipe to grep, if the returned value < 0 an error has occured */
            perror("pipe failed");
            return EXIT_FAILURE;
        }

        greppid = fork();               /* creates a child process for the grep function */
        if (greppid < 0) {              /* if this is TRUE, an error has occured and the program is shut down */
            perror("fork failed");
            return EXIT_FAILURE;
        }

        if (greppid == 0) {             /* if this is TRUE then the program is in the child process */

            grep(argv);                 /* a call to the function grep() */

        }

        pipe_close(pipe_print);         /* closing of pipe after use */

        if (pipe(pipe_sort) < 0) {      /* opening pipe to sort, if the returned value < 0 an error has occured */
            perror("pipe failed");
            return EXIT_FAILURE;
        }

        sortpid = fork();               /* creates a child process for the sort function */
        if (sortpid < 0) {              /* if this is TRUE, an error has occured and the program is shut down */
            perror("fork failed");
            return EXIT_FAILURE;
        }

        if (sortpid == 0) {             /* if this is TRUE then the program is in the child process */

            sort(pipe_grep);            /* a call to the function sort() */

        }

        pipe_close(pipe_grep);          /* closing of pipe after use */

        ret = waitpid(greppid, &status, 0);         /* this makes the program wait for grep to finish to avoid bypassing grep */
        if( ret < 0 || status != 0) {               /* if waitpid() fails or the child process returns a status != 0 an error has occured */
            perror( "waitpid() for grep failed" );
            exit( 1 );
        }

    } else {                            /* this part executes if no parameters to grep is entered */

        if (pipe(pipe_sort) < 0) {      /* opening pipe to sort, if the returned value < 0 an error has occured */
            perror("pipe failed");
            return EXIT_FAILURE;
        }

        sortpid = fork();               /* creates a child process for the sort function */
        if (sortpid < 0) {              /* if this is TRUE, an error has occured and the program is shut down */
            perror("fork failed");
            return EXIT_FAILURE;
        }

        if (sortpid == 0) {             /* if this is TRUE then the program is in the child process */

            sort(pipe_print);           /* a call to the function sort() */

        }

        pipe_close(pipe_print);         /* closing of pipe after use */
    }

    pagerpid = fork();                  /* creates a child process for the pager function */
    if (pagerpid < 0) {                 /* if this is TRUE, an error has occured and the program is shut down */
        perror("fork failed");
        return EXIT_FAILURE;
    }

    if (pagerpid == 0) {                /* if this is TRUE then the program is in the child process */

        pager();                        /* a call to the function pager() */

    }

    pipe_close(pipe_sort);              /* closing of pipe after use */

    ret = waitpid(printenvpid, &status, 0);         /* this makes the program wait for printenv to finish to avoid bypassing printenv */
    if( ret < 0 || status != 0) {                   /* if waitpid() fails or the child process returns a status != 0 an error has occured */
        perror( "waitpid() for printenv failed" );
        exit( 1 );
    }
    ret = waitpid(sortpid, &status, 0);             /* this makes the program wait for sort to finish to avoid bypassing sort */
    if( ret < 0 || status != 0) {                   /* if waitpid() fails or the child process returns a status != 0 an error has occured */
        perror( "waitpid() for sort failed" );
        exit( 1 );
    }
    ret = waitpid(pagerpid, &status, 0);            /* this makes the program wait for pager to finish to avoid bypassing pager */
    if( ret < 0 || status != 0) {                   /* if waitpid() fails or the child process returns a status != 0 an error has occured */
        perror( "waitpid() for pager failed" );
        exit( 1 );
    }

    exit(0);
}