Why does typing "./" in sh produce "permission denied"?

If I were to guess, your sh favours simplicity or performance over user friendliness. The "permission denied" error is that provided by perror(3), a standard function for printing an error message. For example:

$ cat foo.c #include <stdio.h> #include <unistd.h> int main() { char* const args[] = { "/usr", NULL }; if (execv(args[0], args)) perror(args[0]); return 0; } $ gcc -o foo foo.c $ ./foo /usr: Permission denied 

bash probably makes a check to see if the path is a directory. That will, of course, be slightly slower and be slightly longer code.

bash, zsh, etc. have more than one reason to make the check - they allow you to "run" a path to a directory to cd to it:

$ shopt -s autocd $ cd / $ pwd / $ /usr/share cd /usr/share $ pwd /usr/share 

In the case of dash (Debian's /bin/sh points to /bin/dash), that's pretty much the case. The code that executes the command is in shellexec():

if (strchr(argv[0], '/') != NULL) { tryexec(argv[0], argv, envp); e = errno; } else { // snip exerror(EXEXIT, "%s: %s", argv[0], errmsg(e, E_EXEC)); 

That function calls errms():

const char * errmsg(int e, int action) { if (e != ENOENT && e != ENOTDIR) return strerror(e); if (action & E_OPEN) return "No such file"; else if (action & E_CREAT) return "Directory nonexistent"; else return "not found"; } 

strerror(3) is another standard function, like perror. strerror returns the error message, perror prints it directly.