Shell Programming and Scripting

I see lot of ad-hoc shell scripts in our servers which don't have a shebang at the beginning .

Does this mean that it will run on any shell ?

Is it a good practice to create scripts (even ad-hoc ones) without shebang ?

They will run in the shell of the user who executed it.
So if user uses /bin/bash, the script will be executed in that shell.

No, that does not mean it will work in every shell, it probably won't.

That's incorrect. When an executable file's header/magic isn't recognized, it is passed to /bin/sh regardless of the user's shell.

Note that this behavior (falling back on /bin/sh) is only available through some library functions (execlp, execvp, execvpe) which wrap the execve(2) system call. Any code that tries to execute such a script using other means (such as directly invoking execve(2)) will fail.

Regards,
Alister

Note that if the script is sourced (e.g. . /path/to/script.sh) rather than run as an executable file then it will be run in the current shell.

Is that correct?
I'm on FreeBSD (sorry don't know how to get version info). The login uses bash.
If I run a script containing only this line

Code:

[[ 1 == 1 ]] && echo yes || echo no

It results in (seems to use bash)

Code:

yes

If I enter a sh-shell with sh and run the script I get (seems to use sh)

Code:

[[: not found
no

True.

With regard to the original post, if the script is sourced, even if the shebang is present, it is never used.

Regards,
Alister

---------- Post updated at 10:02 AM ---------- Previous update was at 09:39 AM ----------

It should be correct. It's a fundamental and standardized behavior. From your description, it sounds like you are explicitly passing the script to different shells, but I can't be certain. You did not clearly and unambiguously state the exact commands that you used to create your script and run it. Please do so.

For version info, you should be able to get it from uname -a.

Regards,
Alister

---------- Post updated at 10:24 AM ---------- Previous update was at 10:02 AM ----------

The following excerpts are from the current version of FreeBSD's libc. The most relevant bits have been highlighted:

From FreeBSD CVS - src/lib/libc/gen/exec.c

Code:

retry:		(void)_execve(bp, argv, envp);
		switch (errno) {
		case E2BIG:
			goto done;
		case ELOOP:
		case ENAMETOOLONG:
		case ENOENT:
			break;
		case ENOEXEC:
			for (cnt = 0; argv[cnt]; ++cnt)
				;
			memp = alloca((cnt + 2) * sizeof(char *));
			if (memp == NULL) {
				/* errno = ENOMEM; XXX override ENOEXEC? */
				goto done;
			}
			memp[0] = "sh";
			memp[1] = bp;
			bcopy(argv + 1, memp + 2, cnt * sizeof(char *));
 			(void)_execve(_PATH_BSHELL,
			    __DECONST(char **, memp), envp);
			goto done;

execvpe(3) calls execve(2) (through a libc wrapper). If the system call fails with ENOEXEC, that means that an executable file was found but its format is not recognized. This happens if the file is a foreign binary or if it's a shell script without a shebang. execvpe(3) then reattempts to execve a bourne shell with the unrecognized file as its first argument.

From FreeBSD CVS - src/include/paths.h

Code:

#define	_PATH_BSHELL	"/bin/sh"

The path to the bourne shell is hardcoded in a macro.

I also took a look at glibc and they too share the same implementation design. execlp and execvp wrap execvpe. execvpe takes care of checking errno after execve, retrying with /bin/sh if appropriate. They both use _PATH_BSHELL to point to /bin/sh.

For glibc, the relevant files:
sourceware.org Git - glibc.git/blob - posix/execvpe.c
sourceware.org Git - glibc.git/blob - sysdeps/unix/sysv/linux/paths.h

Regards,
Alister

Thank you Alister. Thanks everyone.

I have 4 quick questions:

Question 1.

According to Wikipedia /bin/sh means Bourne shell . Is that Right?

Shebang (Unix) - Wikipedia, the free encyclopedia


Question2.
So, in Solaris 10, RHEL Version5, version 6, AIX version 6, 7 , if i forget to add shebang to a script , will the script be executed using Bourne shell ?


Question3.
Regarding CarloM's post on sourcing . /path/to/script.sh
I have used sourcing only to set environmental variables in the current shell. Can sourcing be used to execute shell scripts as well?



Question4.
I have noticed that some people execute shell scripts by putting sh at the beginning like

Code:

sh /path/to/myscript.sh

I always execute shell scripts without the sh at the beginning.

Code:

/path/to/script.sh

Which is recommended ?