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Why does bash seem to parse `sh -c` commands differently when called via `execl`?
When I do this in a shell:
$ /bin/sh -c 'echo hello world'
hello world
it's my understanding that I'm running a process with argv = {"/bin/sh", "-c", "echo hello world"}. That is, the quotes cause all three words to be passed as one argument to sh, and that instance of sh is responsible for splitting it on spaces before executing it.
I'm trying to have a C program do the same thing. I tried this:
#include <unistd.h>
int main() {
execl("/bin/sh", "-c", "echo hello world", NULL);
}
but for some reason the shell seems to parse its arguments differently when called this way:
$ ./a.out
-c: echo hello world: No such file or directory
That is, it's getting argv[2] = "echo hello world", and rather than parsing that as a shell command, deciding that must all be the argv[0] of the subprocess it's been asked to start.
My /bin/sh is bash. Using /bin/bash explicitly, or /bin/zsh, does the same thing, but busybox sh seems to do what I expect:
$ cat /tmp/what.c
#include <unistd.h>
int main() {
execl("/bin/busybox", "sh", "-c", "echo hello world", NULL);
}
$ gcc /tmp/what.c && ./a.out
hello world
Why (and for that matter, how) do bash and zsh behave this way? How can I get them to not do that?
1 answer
Because execl's first argument isn't argv[0].
execl("/bin/sh", "-c", ...) runs the executable /bin/sh, but sets its argv[0] to "-c", as if one had created a link called -c pointing at it and run that. The correct way to do this is execl("/bin/sh", "sh", "-c", ...).
Busybox works here by accident, because it has sh as its argv[0]: its argv when executed as I did in my last example would be {"sh", "-c", "echo hello world"}. Busybox, specifically, actually does support being called like that (a normal way to use busybox is to install it somewhere and then symlink all the commands it supports, including sh, to it).
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