Hello All,
This post details how to write your own minimal shell. The source code is pasted below.
Makefile:
——————–
all:
g++ -o /bin/doit doit.cpp
Source Code: doit.cpp
————–
/*
** System Programming Lab
** Program: doit-1.2 shell
** File: doit.cpp
** Author: Zobayer Hasan (3rd year, Roll-1591)
** Date: May 8, 2011
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <time.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <sys/resource.h>
// flag for enabling / disabling status display
int __SHOW_DETAILS__ = 0;
int __CURR_BCKGRND__ = 0;
struct Node {
rusage ru;
timeval st, en;
char name[128];
int pid, done, sl;
Node *next;
} *head, *tail;
// functions used for our shell ‘DOIT’
void add(Node);
void del(int);
void find(int, Node *);
void changeDirectory(char *);
void printWorkingDirectory(void);
void getCurrentDirectory(char *);
void signalHandler(int);
void showCompletedJobs(int);
void showRunningJobs(void);
void printMessage(timeval *, timeval *, rusage *);
int main(int argc, char **argv) {
char argument[128], temp[128];
char *p, sargs[32][128], *pargs[32];
int i, child_status, background;
pid_t pid, wait_pid;
Node b;
timeval st, en;
rusage rs;
signal(SIGCHLD, signalHandler);
signal(SIGINT, SIG_IGN);
if(argc > 1) {
for(i = 1; i < argc; i++) pargs[i-1] = argv[i]; pargs[i-1] = NULL;
pid = fork();
if(pid == -1) {
printf(“doit: system call fork() failed…\n”);
exit(1);
}
if(!pid) {
execvp(pargs[0], pargs);
printf(“doit: %s: command not found…\n”, pargs[0]);
exit(2);
}
else {
gettimeofday(&st, NULL);
if((wait_pid = waitpid(pid, &child_status, 0)) == -1) {
printf(“doit: system call waitpid() failed…\n”);
exit(3);
}
getrusage(RUSAGE_CHILDREN, &rs);
gettimeofday(&en, NULL);
if(__SHOW_DETAILS__) {
printf(“PID %d\t[%s] completed.\n”, wait_pid, pargs[0]);
printMessage(&st, &en, &rs);
}
}
}
else {
while(1) {
getCurrentDirectory(temp);
printf(“doit-1.2 [%s]$ “, temp);
fgets(argument, 128, stdin);
p = strtok(argument, ” \n”);
showCompletedJobs(0);
if(!p) continue;
for(i = 0; p; i++, p = strtok(0, ” \n”)) {
strcpy(sargs[i], p);
pargs[i] = sargs[i];
}
if(i && !strcmp(sargs[i-1], “&”)) { background = 1; i–; }
else background = 0;
pargs[i] = NULL; sargs[i][0] = 0;
if(!strcmp(sargs[0], “exit”)) {
showCompletedJobs(1);
printf(“\ndoit: exiting shell…\n”);
return atoi(sargs[1]);
}
if(!strcmp(sargs[0], “cd”)) {
changeDirectory(sargs[1]);
continue;
}
if(!strcmp(sargs[0], “pwd”)) {
printWorkingDirectory();
continue;
}
if(!strcmp(sargs[0], “jobs”)) {
showCompletedJobs(1);
continue;
}
if(!strcmp(sargs[0], “@stats”)) {
if(!strcmp(sargs[1], “on”)) __SHOW_DETAILS__ = 1;
else if(!strcmp(sargs[1], “off”)) __SHOW_DETAILS__ = 0;
else printf(“Status display: %s\nOptions: [on/off]\n”, (__SHOW_DETAILS__? “ON” : “OFF”));
continue;
}
pid = fork();
if(pid == -1) {
printf(“doit: system call fork() failed…\n”);
exit(1);
}
if(!pid) {
execvp(pargs[0], pargs);
printf(“doit: %s: command not found…\n”, pargs[0]);
exit(2);
}
else {
if(background) {
memset(&b, 0, sizeof(Node));
b.pid = pid; strcpy(b.name, pargs[0]);
gettimeofday(&b.st, NULL); b.done = 1;
b.sl = 1 + __CURR_BCKGRND__;
add(b);
printf(“[%d] %d\t[%s]\n”, b.sl, pid, pargs[0]);
}
else {
gettimeofday(&st, NULL);
if((wait_pid = waitpid(pid, &child_status, 0)) == -1) {
printf(“doit: system call wait() failed…\n”);
exit(4);
}
getrusage(RUSAGE_CHILDREN, &rs);
gettimeofday(&en, NULL);
if(__SHOW_DETAILS__) {
printf(“PID %d\t[%s] completed.\n”, wait_pid, pargs[0]);
printMessage(&st, &en, &rs);
}
}
}
}
}
return 0;
}
void add(Node N) {
if(!head) {
head = new Node;
*head = N;
head->next = NULL;
tail = head;
}
else {
tail->next = new Node;
tail = tail->next;
*tail = N;
tail->next = NULL;
}
}
void del(int pid) {
Node *temp, *curr;
if(!head) return;
if(head->pid == pid) {
temp = head;
head = head->next;
delete temp;
if(head == NULL) {
tail = NULL;
__CURR_BCKGRND__ = 0;
}
return;
}
curr = head;
while(curr->next && curr->next->pid == pid) {
temp = curr->next;
curr->next = temp->next;
delete temp;
return;
}
}
Node * find(int pid) {
Node *curr = head;
while(curr) {
if(curr->pid == pid) return curr;
curr = curr->next;
}
return NULL;
}
void changeDirectory(char *ptr) {
char curr[128];
int ret, i, pos;
getcwd(curr, 128);
if(!ptr[0]) ret = chdir((const char *)getenv(“HOME”));
else if(ptr[0]==’.’ && ptr[1]!=’.') {
strcat(curr, ptr+1);
ret = chdir((const char *)curr);
}
else if(!strcmp(ptr, “..”)) {
for(i = pos = 0; curr[i]; i++) if(curr[i]==’/') pos = i;
if(!pos) pos++; curr[pos] = 0;
ret = chdir((const char *)curr);
}
else if(ptr[0]==’/') ret = chdir((const char *)ptr);
else {
strcat(curr, “/”);
strcat(curr, ptr);
ret = chdir((const char *)curr);
}
if(ret == -1) printf(“doit: %s: No such directory…\n”, ptr);
}
void printWorkingDirectory(void) {
char curr[128];
puts(getcwd(curr, 128));
}
void getCurrentDirectory(char *s) {
char curr[128];
int i, pos;
getcwd(curr, 128);
for(i = pos = 0; curr[i]; i++) if(curr[i] ==’/') pos = i;
if(i == 1) strcpy(s, “/”);
else strcpy(s, &curr[pos+1]);
}
void signalHandler(int sig) {
int status;
pid_t t_pid;
rusage rs;
timeval ts;
Node *n;
if(sig == SIGCHLD) {
t_pid = wait(&status);
if(t_pid > 0) {
getrusage(RUSAGE_CHILDREN, &rs);
gettimeofday(&ts, NULL);
n = find(t_pid);
if(n != NULL) {
n->ru = rs;
n->en = ts;
n->done = 0;
}
}
}
signal(SIGCHLD, signalHandler);
}
void showCompletedJobs(int incR) {
Node *curr = head, *temp;
while(curr) {
temp = curr->next;
if(! curr->done) {
printf(“[%d] %d\t[%s] completed.\n”, curr->sl, curr->pid, curr->name);
if(__SHOW_DETAILS__) {
printMessage(&(curr->st), &(curr->en), &(curr->ru));
}
del(curr->pid);
}
curr = temp;
}
if(incR) showRunningJobs();
}
void showRunningJobs() {
Node *curr = head;
while(curr) {
printf(“[%d] %d\t[%s] Running.\n”, curr->sl, curr->pid, curr->name);
curr = curr->next;
}
}
void printMessage(timeval *st, timeval *en, rusage *rs) {
timeval tu, tv;
int ist, ien;
ist = st->tv_sec * 10000000 + st->tv_usec;
ien = en->tv_sec * 10000000 + en->tv_usec;
tu = rs->ru_utime;
tv = rs->ru_stime;
printf(“\n—Process Statistics—\n”);
printf(“user time = %.3lf(ms); system time = %.3lf(ms); wallclock time = %.3lf(ms)\n”, (tu.tv_sec * 1000000.0 + tu.tv_usec) / 1000.0, (tv.tv_sec * 1000000.0 + tv.tv_usec) / 1000.0, (ien-ist)/1000.0);
printf(“voluntary context switches = %ld; involuntary context switches = %ld\n”, rs->ru_nvcsw, rs->ru_nivcsw);
printf(“total page faults = %ld; minor page faults = %ld\n”,rs->ru_majflt + rs->ru_minflt, rs->ru_minflt);
printf(“————————\n”);
}
/* END OF SOURCE CODE */
Notes:
——–
1. Place the source (doit.cpp) and makefile in same directory.
2. Type make. the output is put in /usr/bin directory
3. just type doit and you will see your shell prompt 
