执行与执行

Question

Is there any occasion in which is better to use execl instead of execlp ? I think that maybe when a program is in two different folders using execlp could lead to confusion but I don't know if it is the only case. I ask because one could think that writing execlp("ls", ...) is easier than writing execl("/bin/ls", ...) .

Answer 1

Security

Looking programs up via PATH is convenient, but it can also be insecure. If a directory in a user's PATH is world writable, it's possible to inject a malicious program into the PATH lookup. This would affect execlp but not execl .

For example, if you had a PATH like /foo/bar/bin:/home/you/bin:/usr/bin:/bin and /foo/bar/bin was world writable, someone with access to that machine could copy a malicious program to /foo/bar/bin/ls . Then executing ls would run /foo/bar/bin/ls rather than /bin/ls . They'd be able to execute commands as you and gain greater access.

For this reason, it's often a good idea to refer to specific executables in known locations. Or to hard wire a secure PATH in the executable.

Compatibility

While there is a common set of Unix commands and features specified by POSIX , many programs rely on extensions. If your program uses those extensions, grabbing the first one in the PATH might not be a good idea.

For example, here on OS X the installed utilities in /bin and /usr/bin are BSD-flavored. But I have GNU versions installed earlier in my PATH . A program designed to run on OS X would want to explicitly use, for example, /bin/ls or /usr/bin/tar to be sure they get a known version of those utilities.

$ /usr/bin/tar --version
bsdtar 2.8.3 - libarchive 2.8.3
$ tar --version
tar (GNU tar) 1.29

Answer 2

Both execl() and execlp() work fine and similarly if your executables are in different folders or in the same folder, but you need to set the $PATH if different folders.

execl() is needed for executing executables (like "ls") from command line as you can't go with execlp() in that case. I added a snapshot below.

#include<stdio.h>
#include<unistd.h>
int main(int argc, char *argv[])
{
        if(argc!=2)
        {
                printf("Usage Msg: ./a.out userdefined_executable \n");
                return;
        }
        //execl(argv[1],argv[1],NULL);//**it works** 
        execlp(argv[1],argv[1],NULL);//**it doesn't work** 
        return 0;
}

//Input will be like this, here "p1" is an user-defined executable.
//xyz@xyz:~/stack_overflow$ ./a.out p1