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void work(char * line)
{
    if(check_cd(line))
        return;
    strcpy(line , buf);
    pid_t pid = fork();
    int status;
    int bol_flag = back_ground_check(line);
    if(pid==0) // child
    {
        char *argo [100];
        char *paths = getenv("PATH" );
        char *token = strtok(paths , ":");
        if(strstr(line, "echo") != NULL) {
            parse_echo(line , argo);
        }
        else
            parse(line , argo);
        if(execv (*argo , argo)>0)
            return;
        do
        {
            char temp[100];
            strcpy(temp , token);
            char temp2[50];
            strcpy(temp2, "/");
            strcat(temp2 , *argo);
            strcat(temp , temp2);
            if(execv(temp, argo) >0)
                break;
            token = strtok(NULL , ":");
        }while(token!= NULL);
        exit(1);
    }
    else if(pid <0)
    {
        exit(1);
    }
    else
    {
        if(!bol_flag)
            while (wait(&status) != pid) ;
    }
}

prog.c: In function 'work':
prog.c:3:8: warning: implicit declaration of function 'check_cd' [-Wimplicit-function-declaration]
     if(check_cd(line))
        ^
prog.c:5:5: warning: implicit declaration of function 'strcpy' [-Wimplicit-function-declaration]
     strcpy(line , buf);
     ^
prog.c:5:5: warning: incompatible implicit declaration of built-in function 'strcpy'
prog.c:5:5: note: include '<string.h>' or provide a declaration of 'strcpy'
prog.c:5:19: error: 'buf' undeclared (first use in this function)
     strcpy(line , buf);
                   ^
prog.c:5:19: note: each undeclared identifier is reported only once for each function it appears in
prog.c:6:5: error: unknown type name 'pid_t'
     pid_t pid = fork();
     ^
prog.c:6:17: warning: implicit declaration of function 'fork' [-Wimplicit-function-declaration]
     pid_t pid = fork();
                 ^
prog.c:8:20: warning: implicit declaration of function 'back_ground_check' [-Wimplicit-function-declaration]
     int bol_flag = back_ground_check(line);
                    ^
prog.c:12:23: warning: implicit declaration of function 'getenv' [-Wimplicit-function-declaration]
         char *paths = getenv("PATH" );
                       ^
prog.c:12:23: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
prog.c:13:23: warning: implicit declaration of function 'strtok' [-Wimplicit-function-declaration]
         char *token = strtok(paths , ":");
                       ^
prog.c:13:23: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
prog.c:14:12: warning: implicit declaration of function 'strstr' [-Wimplicit-function-declaration]
         if(strstr(line, "echo") != NULL) {
            ^
prog.c:14:12: warning: incompatible implicit declaration of built-in function 'strstr'
prog.c:14:12: note: include '<string.h>' or provide a declaration of 'strstr'
prog.c:14:36: error: 'NULL' undeclared (first use in this function)
         if(strstr(line, "echo") != NULL) {
                                    ^
prog.c:15:13: warning: implicit declaration of function 'parse_echo' [-Wimplicit-function-declaration]
             parse_echo(line , argo);
             ^
prog.c:18:13: warning: implicit declaration of function 'parse' [-Wimplicit-function-declaration]
             parse(line , argo);
             ^
prog.c:19:12: warning: implicit declaration of function 'execv' [-Wimplicit-function-declaration]
         if(execv (*argo , argo)>0)
            ^
prog.c:27:13: warning: implicit declaration of function 'strcat' [-Wimplicit-function-declaration]
             strcat(temp2 , *argo);
             ^
prog.c:27:13: warning: incompatible implicit declaration of built-in function 'strcat'
prog.c:27:13: note: include '<string.h>' or provide a declaration of 'strcat'
prog.c:33:9: warning: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
         exit(1);
         ^
prog.c:33:9: warning: incompatible implicit declaration of built-in function 'exit'
prog.c:33:9: note: include '<stdlib.h>' or provide a declaration of 'exit'
prog.c:37:9: warning: incompatible implicit declaration of built-in function 'exit'
         exit(1);
         ^
prog.c:37:9: note: include '<stdlib.h>' or provide a declaration of 'exit'
prog.c:42:20: warning: implicit declaration of function 'wait' [-Wimplicit-function-declaration]
             while (wait(&status) != pid) ;
                    ^

Standard output is empty