How to make a history command - c

Question

I am having trouble creating a child process, and I'm not sure if I have the execvp argument right. Is there a way to fix it so it'll pass correctly?
int execute(char* input) {
int i = 0;
char* shell_argv[MAX_CMD_LINE_ARGS];
memset(shell_argv, 0, MAX_CMD_LINE_ARGS * sizeof(char));
//passing pointer of input and element list 128
int shell_argc = parse(input, shell_argv);
int status = 0;
pid_t pid = fork();
if (pid < 0) {
fprintf(stderr, "Fork() failed\n"); } // send to stderr
else if (pid == 0) { // child
// fill in code for execvp(...) <- this is what I'm having trouble with
if (execvp(shell_argv[0], shell_argv) == -1 && strcmp(input, "history") != 0) {
printf("Invalid command\n");
}
} else { // parent ----- don't wait if you are creating a daemon (background) process
while (wait(&status) != pid) { }
}
return 0;
}

Answer 1

There are some errors in your code:
shell_argv is an array of char*, memset length shoud be MAX_CMD_LINE_ARGS * sizeof(char*); or use a simple way char *shell_argv[MAX_CMD_LINE_ARGS] = {0};
I can't find a standard function parse(), maybe you implemented it by your self. I have made a workround to run the code.
memory for the element in shell_argv should be free at the end of founction.
The input argument of execvp is correct. Here is all the code for your reference.
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#include <stdlib.h>
#define MAX_CMD_LINE_ARGS 255
int parse(char *input, char** shell_argv){
shell_argv[0] = strdup("date");
shell_argv[1] = strdup("+%s");
}
int execute(char *input)
{
int i = 0;
char *shell_argv[MAX_CMD_LINE_ARGS];
memset(shell_argv, 0, MAX_CMD_LINE_ARGS * sizeof(char*));
// passing pointer of input and element list 128
int shell_argc = parse(input, shell_argv);
int status = 0;
pid_t pid = fork();
if (pid < 0)
{
fprintf(stderr, "Fork() failed\n");
} // send to stderr
else if (pid == 0)
{ // child
// fill in code for execvp(...) <- this is what I'm having trouble with
if (execvp(shell_argv[0], shell_argv) == -1 && strcmp(input, "history") != 0)
{
printf("Invalid command\n");
}
}
else
{ // parent ----- don't wait if you are creating a daemon (background) process
while (wait(&status) != pid)
{
}
}
for(int i=0;i<MAX_CMD_LINE_ARGS;i++){
if(shell_argv[i] != NULL){
free(shell_argv[i]);
}
}
return 0;
}
int main(){
execute("date +%s");
return 0;
}
Test result:
gxie#ubuntu20:~/test $ gcc main.c
gxie#ubuntu20:~/test $ ./a.out
1663837441