How to use execvp()

The user will read a line and i will retain the first word as a command for execvp.

Lets say he will type "cat file.txt" ... command will be cat . But i am not sure how to use this execvp(), i read some tutorials but still didn't get it.

#include <stdio.h>
#include <stdlib.h>
int main()
{ char *buf; char command[32]; char name[32]; char *pointer; char line[80]; printf(">"); while((buf = readline(""))!=NULL){ if (strcmp(buf,"exit")==0) break; if(buf[0]!=NULL) add_history(buf); pointer = strtok(buf, " "); if(pointer != NULL){ strcpy(command, pointer); } pid_t pid; int status; if ((pid = fork()) < 0) { printf("*** ERROR: forking child process failed\n"); exit(1); } else if (pid == 0) { if (execvp(command, buf) < 0) { printf("*** ERROR: exec failed\n"); exit(1); } } else while (wait(&status) != pid) ; free(buf); printf(">"); }///end While return 0;
}

2 Answers

The first argument is the file you wish to execute, and the second argument is an array of null-terminated strings that represent the appropriate arguments to the file as specified in the man page.

For example:

char *cmd = "ls";
char *argv[3];
argv[0] = "ls";
argv[1] = "-la";
argv[2] = NULL;
execvp(cmd, argv); //This will run "ls -la" as if it were a command

In cpp, you need to pay special attention to string types when using execvp:

#include <iostream>
#include <string>
#include <cstring>
#include <stdio.h>
#include <unistd.h>
using namespace std;
const size_t MAX_ARGC = 15; // 1 command + # of arguments
char* argv[MAX_ARGC + 1]; // Needs +1 because of the null terminator at the end
// c_str() converts string to const char*, strdup converts const char* to char*
argv[0] = strdup(command.c_str());
// start filling up the arguments after the first command
size_t arg_i = 1;
while (cin && arg_i < MAX_ARGC) { string arg; cin >> arg; if (arg.empty()) { argv[arg_i] = nullptr; break; } else { argv[arg_i] = strdup(arg.c_str()); } ++arg_i;
}
// Run the command with arguments
if (execvp(command.c_str(), argv) == -1) { // Print error if command not found cerr << "command '" << command << "' not found\n";
}

Reference:execlp、execvp用法與範例