How to create a file using execl command in c

Question

#include<stdio.h>
#include<unistd.h>
#include<string.h>
#define MAXLINIE 100


main(int argc, char* argv[]) {
    if (fork()==0){

        execl("/bin/> temporar.txt", "/bin/> temporar.txt", ">temporar.txt", NULL);

    }
}

Basically, what I am trying to do is creating a file using a process in unix, here is my code, but for some reason it does not work, I do not really understand the execl command and why the first two parameters have to be the same: execl("/bin/ls", "/bin/ls", "-l", NULL); this is working well, could someone help me ?

Thanks a lot!

All that parameters will be passed as variardic arguments, that have `void*` type. Anyway, C accepts some kind of implicit conversion, so `NULL` is OK even for `char*`. — loentar, Apr 14 '13 at 11:06
@Ioentar: Verbatim from `man execl`: `The list of arguments must be terminated by a NULL pointer, and, since these are variadic functions, this pointer must be cast (char *) NULL.` — alk, Apr 14 '13 at 11:08
You're right. That is written in `man execl`, that it should be idealogicaly. But this is not a problem for C. Even more no one casts `(char*)NULL` even execl implementation. If you compile `execl(..., NULL)` with maximum level of warning enabled you will not get any warning regarding to implicit cast to `char*`. — loentar, Apr 14 '13 at 11:17
1st parameter to `execl` is the name of the program and then you start arguments to the program as subsequent arguments... The argv[0] to any C program is the name of the program itself. Hence the first two arguments are the same... — Sam, Apr 14 '13 at 11:32

loentar · Answer 1 · 2013-04-14T11:10:22.280

2

Consider use system() instead:

system("/bin/ls -l > temporar.txt");

Or using execl call /bin/sh to redirect stream:

execl("/bin/sh", "/bin/sh", "-c" , "/bin/ls -l >temporar.txt", NULL);

First parameter of execl is a command to execute, second is a first parameter to be passed to command (argv[0]), third and next - other arguments argv[1] ...

edited Apr 14 '13 at 11:10

answered Apr 14 '13 at 10:57

loentar

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I am not allowed to use system, i have to create myself the process, i don't want all the "ls -l", only to create the file – JackRobinson Apr 14 '13 at 11:00
I get "No such file or directory" error when i run it like this "execl("/bin/sh", "/bin/sh", "/bin/>temporar.txt", NULL);" because i only want to create a file in the folder from where I am launching the c source, the ls-l was an example that worked and I've tried to adapt it to my problem – JackRobinson Apr 14 '13 at 11:06
Sorry, forgot to pass -c flag – loentar Apr 14 '13 at 11:10

Grijesh Chauhan · Accepted Answer · 2013-04-14T11:30:13.917

2

first search whereis is touch:

~$ whereis touch
touch: /bin/touch /usr/bin/touch /usr/bin/X11/touch

use: int execl(const char *path, const char *arg, ...);

execl("/bin/touch", "touch", "filename", NULL);
          ^            ^         ^         ^
       command       command   argument    
        path           name
                     arg 0     arg 1

edited Apr 14 '13 at 11:30

answered Apr 14 '13 at 11:03

Grijesh Chauhan

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How to create a file using execl command in c

2 Answers2