I'm creating my own Shell and I successfully got processes to run in the background by using my is_background function to find a &. It was working fine until i tried to implement redirection of standard output. The chk_if_output function is a part of this as well as the if statement if(out[0] == 1) in the process function. Somehow implementing redirection screwed up the way I implemented background process. If I comment out the redirection code it works again. I get a segmentation fault every time I try to run a background process with the redirection code in the program and I can't for the life of me figure out why. I haven't changed any of the background process code.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <fcntl.h>
#define MAX_LINE 80 /* The maximum length command */
int is_background(char *args[], int size){
int background = 0;
if (strcmp(args[size-1 ], "&") == 0){
background = 1;
args[size-1] = NULL;
}
return background;
}
int * chk_if_output(char *args[], int size){
int * out = malloc(2);
out[0] = 0; out[1] = 0;
for (int i = 0; i < size; i++){
if (strcmp(args[i],">") == 0){
out[0] = 1;
out[1] = i;
break;
}
}
return out;
}
void process(char *command, char *params[], int size){
pid_t pid;
int background = is_background(params, size);
int *out = chk_if_output(params, size);
int fd;
int fd2;
pid = fork();
if (pid < 0) {
fprintf(stderr, "Fork Failed\n");
}else if (pid == 0) {
if(out[0] == 1){
for (int i = out[1]; i < size; i++){
params[i] = params[i+1];
}
fd = open(params[out[1]-1],O_RDONLY,0);
dup2(fd,STDIN_FILENO);
close(fd);
fd2 = creat(params[out[1]],0644);
dup2(fd2,STDOUT_FILENO);
close(fd2);
out[0] = 0;
out[1] = 0;
}
execvp(command, params);
}else {
if(background == 1){
waitpid(pid, NULL, 0);
}
background = 0;
}
}
int main(void) {
char *args[MAX_LINE/2 + 1]; /* command line arguments */
int should_run = 1; /* flag to determine when to exit program */
while (should_run) {
char *line;
char *endline;
printf("Leyden_osh>");
fgets(line, MAX_LINE*sizeof line, stdin);
if((endline = strchr(line, '\n')) != NULL){
*endline = '\0';
}
if (strcmp((const char *)line,"exit") == 0){
should_run = 0;
}
int i = 0;
args[i] = strtok(line, " ");
do{
args[++i] = strtok(NULL, " ");
}while(args[i] != NULL);
process(args[0], args, i);
fflush(stdout);
return 0;
}
In the chk_if_output() function, the last element of the array in the loop was NULL.
Fixed it by looping to size -1.
Related
I am trying to make a tiny shell. My problem is that when I call execvp() - I get errors.
For example, when I type in ls -l it returns ls: invalid option -- '
Can someone, please, help me understand why I am getting this error? For my code, the function command split gets the user input, and splits them up into separate commands. Separate commands are seperated by ; character.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <unistd.h>
#define MAX_CHARACTERS 512
#define HISTORY_SIZE 10
int commandSplit(char *c, char *a[], int t[]) {
int count = 0;
int total = 0;
char *temp[MAX_CHARACTERS];
char *readCommands = strtok(c, ";");
while(readCommands != NULL) {
printf("Reading full command: %s\n", readCommands);
temp[count] = readCommands;
count++;
readCommands = strtok(NULL, ";");
}
printf("Done reading full commands\n");
for(int i = 0; i < count; i++) {
char *read = strtok(temp[i], " ");
int track = 0;
while(read != NULL) {
printf("Reading individual command: %s\n", read);
a[total] = read;
track++;
total++;
read = strtok(NULL, " ");
}
t[i] = track;
}
return count;
}
int main() {
int exitProgram = 0;
char *args[MAX_CHARACTERS];
while(!exitProgram) {
char *commands = (char *)(malloc(MAX_CHARACTERS*sizeof(char)));
int tracker[MAX_CHARACTERS];
int numOfCommands = 0;
printf("tinyshell> ");
fgets(commands, MAX_CHARACTERS, stdin);
if(strlen(commands) == 0) continue;
numOfCommands = commandSplit(commands, args, tracker);
printf("There are %i commands!\n", numOfCommands);
if(strcmp(args[0], "exit") == 0) {
printf("Exiting\n");
exitProgram = 1;
continue;
}
int l = 0;
for(int i = 0; i < numOfCommands; i++) {
int status;
char *holder[tracker[i]+1];
for(int j = 0; j < tracker[i]; j++) {
holder[j] = args[l];
printf("Assiging holder:%s\n", holder[j]);
l++;
}
holder[tracker[i]] = NULL;
printf("What is holder? \n");
for(int o = 0; o < tracker[i]; o++) printf("%s", holder[o]);
pid_t p = fork();
pid_t waiting;
if(p == 0) {
printf("I am in child process\n");
execvp(holder[0], holder);
fprintf(stderr, "Child process could not execvp!\n");
exit(1);
}
else {
if(p < 0) {
fprintf(stderr, "Fork FAILED!\n");
}
else {
waiting = wait(&status);
printf("Child %d, status %d\n", waiting, status);
}
}
for(int i = 0; i < numOfCommands; i++) {
args[i] = NULL;
}
}
}
return 0;
}
Your problem is that fgets() also reads the newline character. As a result, the last argument of execvp() arguments array contains a newline, causing ls complain about an unrecognized argument: what you acctually pass to ls is -l\n; what you need to pass is just -l without the newline.
Try adding this code after the fgets call to trim the input buffer:
int len;
len = strlen(commands);
if (len > 0 && commands[len-1] == '\n') {
commands[len-1] = '\0';
}
I am experimenting with writing a simple shell to support all the usual functions.
So far, despite my haphazard approach to it, I have been met with success - I have been able to fork() new processes with parameters but other certain methods don't seem to run.
When I run my shell, functions like pwd, ls, help work whilst other functions such as cd, mkdir don't - why is this & what research/investigation can I do in order to begin to address this?
I've included my code so far (although I'm not sure if it will actually help).
Thanks very much
EDIT: My output, when my shell is running in cygwin, if I type "cd /cygdrive/c" the output is "Command not found"
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
const int MAX_ARGS = 9;
size_t nBytes = 64; //Data type representing size of objects (unsigned)
int bytesRead = -1;
char *cmd = NULL;
char prompt[] = "DaSh-> ";
int argc;
char **argv;
int pid;
int childpid;
int status;
void process();
int readcmd();
int main() {
setvbuf(stdout, NULL, _IONBF, 0);
setvbuf(stderr, NULL, _IONBF, 0);
while (1) {
bytesRead = -1;
printf(prompt);
while (bytesRead == -1) {
bytesRead = readcmd();
}
process(cmd); //process the single line input into arguments
if (strcmp(argv[0], "exit") == 0) { //If user types exit
exit(0);
}
childpid = fork();
if (childpid == 0) { //This is run by the child
execvp(argv[0], argv);
printf("Command not recognised\n");
exit(1);
}
else if (childpid > 0 ) {
waitpid(-1, &status, 0);
}
}
return 0;
}
int nrows = 10;
int ncolumns = 2;
void process(char argStr[]) {
argv = malloc(MAX_ARGS * sizeof(char *)); //Array of pointers to the char first letter of each argument
char delims[] = " \n"; //Delimit about space
int i = 0;
argv[i] = strtok(argStr, delims);
while (argv[i] != NULL) {
argv[++i] = strtok( NULL, delims);
}
argc = i;
}
int readcmd() {
return getline(&cmd, &nBytes, stdin);
}
I'm trying to write a very very simple unix shell in C, and I have the basics of what I need working, except support for a history command. I have a global 2D char array that holds the history of all entered commands. Commands are added before the fork() system call, and I was originally printing out the value of the history global array after strings were added, and they were printing out correctly, so I'm not sure why it doesn't print out when the command "history" is used at the shell.
Thank to anyone who takes a look.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include "myhistory.h"
int BUFFER_SIZE = 1024;
char history[100][80];
int command_index = 0;
int main(int argc, char *argv[]){
int status = 0;
int num_args;
pid_t pid;
while(1){
char *buffer_input, *full_input;
char command[BUFFER_SIZE];
char *args[BUFFER_SIZE];
printf("myshell> ");
buffer_input = fgets(command, 1024, stdin);
full_input = malloc(strlen(buffer_input)+1);
strcpy(full_input, buffer_input);
if (command_index >= 100) {
command_index = 0;
}
strncpy(history[command_index], full_input, strlen(full_input) + 1);
command_index += 1;
parse_input(command, args, BUFFER_SIZE, &num_args);
//check exit and special command conditions
if (num_args==0)
continue;
if (!strcmp(command, "quit" )){
exit(0);
}
if(!strcmp(command, "history")){
int i;
fprintf(stderr,"%d\n",(int)pid);
for(i = 0; i < command_index; i++){
fprintf(stdout, "%d: %s\n",i+1,history[command_index]);
}
continue;
}
errno = 0;
pid = fork();
if(errno != 0){
perror("Error in fork()");
}
if (pid) {
pid = wait(&status);
} else {
if( execvp(args[0], args)) {
perror("executing command failed");
exit(1);
}
}
}
return 0;
}
void parse_input(char *input, char** args,
int args_size, int *nargs){
char *buffer[BUFFER_SIZE];
buffer[0] = input;
int i = 0;
while((buffer[i] = strtok(buffer[i], " \n\t")) != NULL){
i++;
}
for(i = 0; buffer[i] != NULL; i++){
args[i] = buffer[i];
}
*nargs = i;
args[i] = NULL;
}
Change:
fprintf(stdout, "%d: %s\n",i+1,history[command_index]);
to:
fprintf(stdout, "%d: %s\n",i+1,history[i]);
I'm trying to get a better understanding of pipes and processes. I want to implement multiple chained pipes like cat test.txt | sort | uniq -c. I started my code with the cat test.txt, but it isn't working. It compiles, but when I provide a file name in the command line, for example, ./hwk ./test.txt. Nothing returns. Can someone take a look and give me some hints? I want to use loops because I want to be able to add more pipes. I know there's a lot of issues in my code, so I hope someone can give me some guidance on this topic. Thanks.
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#define SIZE 1024
int main (int argc, char **argv)
{
int num_pipe = 1;
int commands = num_pipe + 1; //number of commands is one more than the number of pipes
int fds[num_pipe * 2];
int status;
pid_t pid;
char *str_ptr;
//Pass Command
char *arrayOfCommands[] = {"cat", NULL};
//Setting up pipes
int i;
for (i = 0; i < num_pipe; i++){
if(pipe(fds + i * 2) == -1) {
perror("Error creating pipes");
exit(1);
}
}
int j = 0;
for (i = 0; i < commands - 1; ++i) {
pid = fork();
if (pid == 0) {
if (i < commands) {
if (dup2(fds[j+1], 1) < 0) {
perror("dup2 error");
exit(EXIT_FAILURE);
}
}
if (j != 0) {
if(dup2(fds[j-2], 0) < 0) {
perror("dup2 error");
exit(EXIT_FAILURE);
}
}
for (i = 0; i < 2*num_pipe; i++) {
close(fds[i]);
}
if (execvp(arrayOfCommands[0], arrayOfCommands) < 0) {
perror("Array error");
exit(EXIT_FAILURE);
}
}
else if (pid < 0){
perror("Error");
exit(EXIT_FAILURE);
}
j += 2;
}
for (i = 0; i < 2 * num_pipe; i++){
close(fds[i]);
}
for (i = 0; i < num_pipe + 1; i++) {
wait(&status);
}
return 0;
}
I called this mainly minor adaptation of your program p3.c, compiling it to produce p3. Since there's only one command (cat) being invoked, I juggled things so that it will work correctly. When run as ./p3 p3.c, it prints out the content of the source code.
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
static void err_exit(const char *str);
int main (int argc, char **argv)
{
int num_pipe = 0; // Just cat - no pipes
int commands = num_pipe + 1; // Number of commands is one more than the number of pipes
int fds[num_pipe * 2 + 1]; // Avoid size 0 array
char *arrayOfCommands[3] = { "cat", NULL, NULL};
if (argc != 2)
err_exit("Missing filename argument");
arrayOfCommands[1] = argv[1];
for (int i = 0; i < num_pipe; i++)
{
if (pipe(fds + i * 2) == -1)
err_exit("Error creating pipes");
}
int j = 0;
for (int i = 0; i < commands; ++i)
{
pid_t pid = fork();
if (pid == 0)
{
printf("%d: %s %s\n", (int)getpid(), arrayOfCommands[0], arrayOfCommands[1]);
fflush(stdout);
if (i < commands-1 && dup2(fds[j+1], 1) < 0)
err_exit("dup2 error");
if (j != 0 && dup2(fds[j-2], 0) < 0)
err_exit("dup2 error");
for (i = 0; i < 2*num_pipe; i++)
close(fds[i]);
execvp(arrayOfCommands[0], arrayOfCommands);
err_exit("Array error");
}
else if (pid < 0)
err_exit("Error");
j += 2;
}
for (int i = 0; i < 2 * num_pipe; i++)
close(fds[i]);
for (int i = 0; i < num_pipe + 1; i++)
{
int status;
pid_t pid = wait(&status);
printf("PID %d exited 0x%.4X\n", (int)pid, status);
}
return 0;
}
static void err_exit(const char *str)
{
perror(str);
exit(EXIT_FAILURE);
}
Check that works for you. Then you'll need to work out how you're going to create a second command. Your arrayOfCommands isn't going to help directly. You'll need another array of strings in some shape or form.
An extension to run cat file | rev. The changes are really quite minor. I created a_cat to handle the cat command, a_rev for the rev command, and a_cmds as the array of commands. It was also necessary to fix a loop on i to a loop on k.
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>
static void err_exit(const char *str);
int main (int argc, char **argv)
{
int num_pipe = 1;
int commands = num_pipe + 1; //number of commands is one more than the number of pipes
int fds[num_pipe * 2 + 1]; // Avoid size 0 array
char *a_cat[3] = { "cat", NULL, NULL};
char *a_rev[2] = { "rev", NULL};
char **a_cmds[] = { a_cat, a_rev };
if (argc != 2)
err_exit("Missing filename argument");
a_cat[1] = argv[1];
for (int i = 0; i < num_pipe; i++)
{
if (pipe(fds + i * 2) == -1)
err_exit("Error creating pipes");
}
int j = 0;
for (int i = 0; i < commands; ++i)
{
pid_t pid = fork();
if (pid == 0)
{
printf("%d: %s\n", (int)getpid(), a_cmds[i][0]);
fflush(stdout);
if (i < commands-1 && dup2(fds[j+1], 1) < 0)
err_exit("dup2 error");
if (j != 0 && dup2(fds[j-2], 0) < 0)
err_exit("dup2 error");
for (int k = 0; k < 2*num_pipe; k++)
close(fds[k]);
execvp(a_cmds[i][0], a_cmds[i]);
err_exit("Array error");
}
else if (pid < 0)
err_exit("Error");
j += 2;
}
for (int i = 0; i < 2 * num_pipe; i++)
close(fds[i]);
for (int i = 0; i < num_pipe + 1; i++)
{
int status;
pid_t pid = wait(&status);
printf("PID %d exited 0x%.4X\n", (int)pid, status);
}
return 0;
}
static void err_exit(const char *str)
{
perror(str);
exit(EXIT_FAILURE);
}
You aren't passing your program's command-line arguments through to the "cat" child process. You initialize arrayOfCommands like so -> char *arrayOfCommands[] = {"cat", NULL}; <- then you pass it as-is to the execvp() function as the second argument.
Okay your first problem is that in the line:
execvp(arrayOfCommands[0], arrayOfCommands);
you are using arrayOfCommands but I am not sure how you're populating arrayOfCommands for the case where the text file is not being displayed. I mean are you setting arrayOfCommands like the following earlier in the code:
char *arrayOfCommands[] = {"cat", "./test.txt", NULL};
If I understand you correctly your program is called hwk and for whatever reason you think ./hwk ./test.txt should be parsed but that means you should be parsing argv.
Okay now that that's out of the way let's look at the bigger problem of how you are setting things up.
So when a shell parses out pipes it does there's quite a bit going on. Consider the following:
foo fooparam1 fooparam2 | bar barparam1 | baz bazparam1 bazparam2
The shell uses recursion to solve the problem:
foo fooparam1 fooparam2 | ( bar barparam1 | baz bazparam1 bazparam2 )
So it would look SOMETHING like:
spawn_sub_pipes(const char *str) {
char *cmd = strtok(str, "|");
char *rest = strtok(NULL, "|");
int fds[2];
pipe(fds[]);
int pid = fork();
if ( pid < 0 ) {
perror("pipe error");
exit(-1);
}
if ( pid ) { /* parent is the writer */
close(fds[0]); /* close reading pipe */
dup2(fds[1], 1); /* we attach stdout to the pipe */
}
if ( pid == 0 ) {
close(fds[1]);
dup2(fds[0], 0); /* attach the pipe to stdin */
if ( rest ) { /* fork next children */
spawn_sub_pipes(rest);
}
execvpe(cmd);
}
}
IMPORTANT NOTE
I have just written the above code out without testing it. Get the idea from it but don't use it verbatim.
When I simply pipe ls -l | sort with this, the program just spits out the results from ls -l infinitely. Can anyone see what's wrong?
Assume that you only have to look at the main function. This will compile, though.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/wait.h>
#define COMMAND_LINE_LENGTH 256
#define HISTORY_LENGTH 10
#define TOKEN_MAX 50
#define DIRECTORY_LENGTH 5
#define DIRECTORY_PREFIX "/bin/"
struct prog_def
{
//Binary location
char *bin;
//Is this program expecting a pipe?
int expecting_pipe;
//Arguments
char *args[TOKEN_MAX + 1];
pid_t pid;
} prog_def;
int get_prog_defs(const char* buf, struct prog_def prog_defs[])
{
char *line = malloc(strlen(buf) + 1);
int prog_count = 0;
char* token;
strcpy(line, buf);
line[strlen(buf)] = 0;
while(1)
{
int arg_count = 0;
//The first time through we have to pass the line
token = strtok(line, " ");
//Each subsequent call we have to pass NULL
//http://www.cplusplus.com/reference/cstring/strtok/
line = NULL;
//Start building the binary location string
prog_defs[prog_count].bin = (char*)malloc(strlen(token) + DIRECTORY_LENGTH + 1);
//Concatenate the directory prefix and command name
strcat(prog_defs[prog_count].bin, DIRECTORY_PREFIX);
strcat(prog_defs[prog_count].bin, token);
//The first argument execvp will expect is the binary location itself
//Redundant but if I wasn't too lazy to read the doc then I'd know why
prog_defs[prog_count].args[arg_count++] = prog_defs[prog_count].bin;
while(1)
{
prog_defs[prog_count].expecting_pipe = 0;
//Check next token for end, pipe, IO redirection, or argument
token = strtok(NULL, " ");
//If we've consumed all tokens
if (token == NULL)
break;
//Pipe
if (strcmp(token, "|") == 0)
{
prog_defs[prog_count - 1].expecting_pipe = 1;
break;
}
//Regular argument
prog_defs[prog_count].args[arg_count++] = token;
}
++prog_count;
if (token == NULL) break;
}
return prog_count;
}
int main(int argc, char** argv)
{
char command[COMMAND_LINE_LENGTH] = {0};
//Generic loop counter
int x = 0;
while(1)
{
printf(">");
//Get the command
gets(command);
struct prog_def prog_defs[TOKEN_MAX];
int prog_count = get_prog_defs(command, prog_defs);
//Keep the previous out fd for the in of the subsequent process
int prev_out_fd = open("/dev/null", O_RDONLY);
for (x = 0; x < prog_count; ++x)
{
//Create a pipe for both processes to share
int pipefd[2];
if (x != prog_count -1)
{
pipe(pipefd);
}
prog_defs[x].pid = fork();
if(prog_defs[x].pid == 0)
{
dup2(prev_out_fd, STDIN_FILENO);
close(pipefd[1]);
if(x != prog_count - 1)
{
dup2(pipefd[1], STDOUT_FILENO);
close(pipefd[0]);
close(pipefd[1]);
}
execvp(prog_defs[x].bin, prog_defs[x].args);
prev_out_fd = pipefd[0];
close(pipefd[1]);
}
close(prev_out_fd);
prev_out_fd = pipefd[0];
close(pipefd[1]);
}
printf("\n");
for (x = 0; x < prog_count; ++x)
{
waitpid(prog_defs[x].pid, NULL, 0);
}
}
}
You call malloc to get some memory for a string, which will be uninitialized, so contain random garbage. You then call strcat which will attempt to append another string to the random garbage and almost certainly run off the end of the malloc'd space, leading to random confusing behavior and crashes.
You also use prog_defs[prog_count - 1] before ever incrementing prog_count, so the first time through the loop, (when prog_count == 0) this will write before the start of the array, which also leads to random confusing behavior and crashes.