Exec

execl()

execl() receives the location of the executable file as its first argument. The next arguments will be available to the file when it’s executed. The last argument has to be NULL:

int execl(const char *pathname, const char *arg, ..., NULL)

Example:

#include <unistd.h> 
 
int main(void) 
{ 
	char *file = "/usr/bin/echo"; 
	char *arg1 = "Hello world!"; 
	execl(file, file, arg1, NULL); 
	return 0; 
}

execlp()

execlp() is very similar to execl(). However, execlp() uses the PATH environment variable to look for the file. Therefore, the path to the executable file is not needed:

int execlp(const char *file, const char *arg, ..., NULL)

Example:

#include <unistd.h> 
 
int main(void) 
{ 
	char *file = "echo"; 
	char *arg1 = "Hello world!";
	execlp(file, file, arg1, NULL); 
	return 0; 
}

execle()

If we use execle(), we can pass environment variables to the function, and it’ll use them:

int execle(const char *pathname, const char *arg, ..., NULL, char *const envp[])

Example:

#include <unistd.h> 
 
int main(void) 
{ 
	char *file = "/usr/bin/bash"; 
	char *arg1 = "-c"; 
	char *arg2 = "echo $ENV1 $ENV2!"; 
	char *const env[] = {"ENV1=Hello", "ENV2=World", NULL}; 
	execle(file, file, arg1, arg2, NULL, env); 
	return 0; 
}

execv()

execv(), unlike execl(), receives a vector of arguments that will be available to the executable file. In addition, the last element of the vector has to be NULL:

int execv(const char *pathname, char *const argv[])

Example:

#include <unistd.h> 
 
int main(void) 
{ 
	char *file = "/usr/bin/echo"; 
	char *const args[] = {"/usr/bin/echo", "Hello world!", NULL}; 
	execv(file, args); 
	return 0; 
}

execvp()

Just like execlp(), execvp() looks for the program in the PATH environment variable:

int execvp(const char *file, char *const argv[])

Example:

#include <unistd.h> 
 
int main(void) 
{ 
	char *file = "echo"; 
	char *const args[] = {"/usr/bin/echo", "Hello world!", NULL}; 
	execvp(file, args); 
	return 0; 
}

execve()

We can pass environment variables to execve(). In addition, the arguments need to be inside a NULL-terminated vector:

int execve(const char *pathname, char *const argv[], char *const envp[])

Example:

#include <unistd.h> 
 
int main(void) 
{ 
	char *file = "/usr/bin/bash"; 
	char *const args[] = {"/usr/bin/bash", "-c", "echo Hello $ENV!", NULL}; 
	char *const env[] = {"ENV=World", NULL}; 
	execve(file, args, env); 
	return 0; 
}