Should I use a Shebang with Bash scripts? - bash

Question

I am using Bash
$ echo $SHELL
/bin/bash
and starting about a year ago I stopped using Shebangs with my Bash scripts. Can
I benefit from using #!/bin/sh or #!/bin/bash?
Update: In certain situations a file is only treated as a script with the
Shebang, example
$ cat foo.sh
ls
$ cat bar.sh
#!/bin/sh
ls
$ file foo.sh bar.sh
foo.sh: ASCII text
bar.sh: POSIX shell script, ASCII text executable

Answer 1

On UNIX-like systems, you should always start scripts with a shebang line. The system call execve (which is responsible for starting programs) relies on an executable having either an executable header or a shebang line.
From FreeBSD's execve manual page:
The execve() system call transforms the calling process into a new
process. The new process is constructed from an ordinary file, whose
name is pointed to by path, called the new process file.
[...]
This file is
either an executable object file, or a file of data for an interpreter.
[...]
An interpreter file begins with a line of the form:
#! interpreter [arg]
When an interpreter file is execve'd, the system actually execve's the
specified interpreter. If the optional arg is specified, it becomes the
first argument to the interpreter, and the name of the originally
execve'd file becomes the second argument
Similarly from the Linux manual page:
execve() executes the program pointed to by filename. filename must be
either a binary executable, or a script starting with a line of the
form:
#! interpreter [optional-arg]
In fact, if a file doesn't have the right "magic number" in it's header, (like an ELF header or #!), execve will fail with the ENOEXEC error (again from FreeBSD's execve manpage):
[ENOEXEC] The new process file has the appropriate access
permission, but has an invalid magic number in its
header.
If the file has executable permissions, but no shebang line but does seem to be a text file, the behaviour depends on the shell that you're running in.
Most shells seem to start a new instance of themselves and feed it the file, see below.
Since there is no guarantee that the script was actually written for that shell, this can work or fail spectacularly.
From tcsh(1):
On systems which do not understand the `#!' script interpreter conven‐
tion the shell may be compiled to emulate it; see the version shell
variable. If so, the shell checks the first line of the file to see if
it is of the form `#!interpreter arg ...'. If it is, the shell starts
interpreter with the given args and feeds the file to it on standard
input.
From FreeBSD's sh(1):
If the program is not a normal executable file (i.e., if it
does not begin with the “magic number” whose ASCII representation is
“#!”, resulting in an ENOEXEC return value from execve(2)) but appears to
be a text file, the shell will run a new instance of sh to interpret it.
From bash(1):
If this execution fails because the file is not in executable format,
and the file is not a directory, it is assumed to be a shell script, a
file containing shell commands. A subshell is spawned to execute it.
You cannot always depend on the location of a non-standard program like bash. I've seen bash in /usr/bin, /usr/local/bin, /opt/fsf/bin and /opt/gnu/bin to name a few.
So it is generally a good idea to use env;
#!/usr/bin/env bash
If you want your script to be portable, use sh instead of bash.
#!/bin/sh
While standards like POSIX do not guarantee the absolute paths of standard utilities, most UNIX-like systems seem to have sh in /bin and env in /usr/bin.

Answer 2

Scripts should always begin with a shebang line. If a script doesn't start with this, then it may be executed by the current shell. But that means that if someone who uses your script is running a different shell than you do, the script may behave differently. Also, it means the script can't be run directly from a program (e.g. the C exec() system call, or find -exec), it has to be run from a shell.

Answer 3

You might be interested in an early description by Dennis M Ritchie (dmr) who invented the #! :
From uucp Thu Jan 10 01:37:58 1980
.>From dmr Thu Jan 10 04:25:49 1980 remote from research
The system has been changed so that if a file
being executed begins with the magic characters #! , the rest of the
line is understood to be the name of an interpreter for the executed
file. Previously (and in fact still) the shell did much of this job;
it automatically executed itself on a text file with executable mode
when the text file's name was typed as a command. Putting the facility
into the system gives the following benefits.
1) It makes shell scripts more like real executable files, because
they can be the subject of 'exec.'
2) If you do a 'ps' while such a command is running, its real name
appears instead of 'sh'. Likewise, accounting is done on the basis of
the real name.
3) Shell scripts can be set-user-ID.
4) It is simpler to have alternate shells available; e.g. if you like
the Berkeley csh there is no question about which shell is to
interpret a file.
5) It will allow other interpreters to fit in more smoothly.
To take advantage of this wonderful opportunity, put
#! /bin/sh
at the left margin of the first line of your shell scripts. Blanks
after ! are OK. Use a complete pathname (no search is done). At the
moment the whole line is restricted to 16 characters but this limit
will be raised.
Hope this helps

Answer 4

If you write bash scripts, i.e. non portable scripts containing bashisms, you should keep using the #!/bin/bash shebang just to be sure the correct interpreter is used. You should not replace the shebang by #!/bin/sh as bash will run in POSIX mode so some of your scripts might behave differently.
If you write portable scripts, i.e. scripts only using POSIX utilities and their supported options, you might keep using #!/bin/sh on your system (i.e. one where /bin/sh is a POSIX shell).
It you write stricly conforming POSIX scripts to be distributed in various platforms and you are sure they will only be launched from a POSIX conforming system, you might and probably should remove the shebang as stated in the POSIX standard:
As it stands, a strictly conforming application must not use "#!" as the first two characters of the file.
The rationale is the POSIX standard doesn't mandate /bin/sh to be the POSIX compliant shell so there is no portable way to specify its path in a shebang. In this third case, to be able to use the 'find -exec' syntax on systems unable to run a shebangless still executable script, you can simply specify the interpreter in the find command itself, eg:
find /tmp -name "*.foo" -exec sh -c 'myscript "$#"' sh {} +
Here, as sh is specified without a path, the POSIX shell will be run.

Answer 5

The header is useful since it specifies which shell to use when running the script. For example, #!/bin/zsh would change the shell to zsh instead of bash, where you can use different commands.
For example, this page specifies the following:
Using #!/bin/sh, the default Bourne shell in most commercial variants
of UNIX, makes the script portable to non-Linux machines, though you
sacrifice Bash-specific features ...

Answer 6

TL;DR: always in scripts; please not in source'd scripts
Always in your parent
FYI: POSIX compliant is #!/bin/bash, not #!/bin/sh
You want to clarify this so that nothing else overrides the interpreter your script is made for.
You don't want a user at the terminal using zsh to have trouble if your script was written for POSIX bash scripts.
You don't want to run source in your #!/bin/bash unrecognized by #!/bin/sh, someone in an sh terminal have it break the script because it is expecting the simple/POSIX . for including source'd files
You don't want e.g. zsh features - not available in other interpreters - to make their way into your bash code. So, put #!/bin/bash in all your script headers. Then, any of your zsh habits in your script will break so you know to remove them before your roll-out.
It's probably best, especially so POSIX-compliant scripts don't break in a terminal like zsh.
Not expected for included source scripts
FYI: POSIX compliant for sourcing text in a BASH script is ., not source
You can use either for sourcing, but I'll do POSIX.
Standard "shebanging" for all scripting:
parent.sh:
#!/bin/bash
echo "My script here"
. sourced.sh # child/source script, below
sourced.sh:
echo "I am a sourced child script"
But, you are allowed to do this...
sourced.sh: (optional)
#!/bin/bash
echo "I am a sourced child script"
There, the #!/bin/bash "shebang" will be ignored. The main reason I would use it is for syntax highlighting in my text editor. However, in the "proper" scripting world, it is expected that your rolled-out source'd script will not contain the shebang.

Answer 7

In addition to what the others said, the shebang also enables syntax highlighting in some text editors, for example vim.

Answer 8

$SHELL and #!/bin/bash or #!/bin/sh are different.
To start, #!/bin/sh is a symlink to /bin/bash on most Linux systems (on Ubuntu it is now /bin/dash)
But on whether to start with /bin/sh or /bin/bash:
Bash and sh are two different shells. Basically bash is sh, with more
features and better syntax. Most commands work the same, but they are
different.
Just assume if you're writing a bash script, stick with /bin/bash and not /sh because problems can arise.
$SHELL does not necessarily reflect the currently running shell.
Instead, $SHELL is the user's preferred shell, which is typically the
one set in /etc/passwd. If you start a different shell after logging
in, you can not necessarily expect $SHELL to match the current shell
anymore.
This is mine for example, but it could also be /root:/bin/dash or /root:/bin/sh depending on which shell you have input in passwd. So to avoid any problems, keep the passwd file at /bin/bash and then using $SHELL vs. #!/bin/bash wouldn't matter as much.
root#kali:~/Desktop# cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
Sources:
http://shebang.mintern.net/bourne-is-not-bash-or-read-echo-and-backslash/
https://unix.stackexchange.com/questions/43499/difference-between-echo-shell-and-which-bash
http://man.cx/sh
http://man.cx/bash