[Libguestfs] shebang usage in bash scripts

[Richard W.M. Jones]

On Fri, Aug 26, 2022 at 07:29:34AM -0500, Eric Blake wrote:
> On Thu, Aug 25, 2022 at 02:35:47PM +0100, Richard W.M. Jones wrote:
> > On Thu, Aug 25, 2022 at 03:32:15PM +0200, Laszlo Ersek wrote:
> > > Hi,
> > > 
> > > most shell scripts in the v2v projects start with a shebang like this:
> > > 
> > > #!/bin/bash -
> > > 
> > > I *think* I understand the intent of the single hyphen, but (a) it seems unnecessary, (b) even if we insisted, using the double-hyphen separator "--" is much more idiomatic (even though the shell, per POSIX, is supposed to interpret "-" identically to "--").
> > > 
> > > Regarding why I think the hyphen is unnecessary:
> > > 
> > > - setuid shell scripts are not a thing on any platform we (should) care about
> > > 
> > > - the script name is not reinterpreted as an option *anyway*
> > > 
> > > Consider:
> > > 
> > > cat >-v <<EOT
> > > #!/bin/bash
> > > echo hello \$1
> > > EOT
> > > 
> > > chmod +x -- -v
> > > 
> > > PATH=$PWD:$PATH -v world
> 
> More believable as:
> 
> (PATH=$PWD:$PATH; -v world)
> 
> In your style, you are relying on the shell using the just-modified
> PATH setting to look up the location of -v; but POSIX allows both a
> system where the shell does its own lookup (and sticks to the old
> PATH, only passing the new PATH to the child process of the -v it
> found on the old PATH), as well as a system where the shell forks off
> a subprocess, the subprocess modifies environ to set the new PATH, and
> then relies on execvp() or similar to do the lookup on what is already
> the new environ.
> 
> But that's a distraction from your main point, that yes, Linux injects
> "--" to the interpreter for a shebang, to protect against an
> executable script whose name begins with '-'.
> 
> > > 
> > > So glibc and/or the Linux kernel already inserts the "--" option/operand delimiter!
> 
> Shebang interpretation is sometimes by the kernel, and sometimes by
> the shell, and it differs by OS.  Some OS's do word-splitting on the
> rest of the shebang after the first space, some treat the entire rest
> of the line as a single argument.
> 
> For a nice summary of some of the warts of shebang lines, see
> 
> https://unix.stackexchange.com/questions/29608/why-is-it-better-to-use-usr-bin-env-name-instead-of-path-to-name-as-my/29620#29620
> 
> If you are worried about /bin/bash being available on all systems
> where a v2v script will be installed, you probably want '#!/bin/env
> bash' to find the first bash in PATH.  But once you add your first
> whitespace, it is no longer portable to use any others, so while
> '#!/bin/bash -' works, '#!/bin/env bash -' does not.  I find that the
> issue of writing a shebang to use a program that may not always be
> installed in the same absolute location across all OSs (and therefore
> where '#!/bin/env name' is useful) is more common than the issue of
> writing a shebang that wants to pass an explicit '-' or '--'
> end-of-option marker to protect against the script being installed
> under a name like '-c ls'.

Fedora packaging guidelines -- rightly or wrongly -- forbid #!/usr/bin/env:

https://docs.fedoraproject.org/en-US/packaging-guidelines/#_shebang_lines

although apparently RPM rewrites them automatically so perhaps we
would not notice.

> Note that GNU coreutils recently added '/bin/env -S', modeled after a
> similar feature first in BSD systems, which makes it possible to write
> '#!/bin/env -S app --with args' parsed in a more similar manner
> regardless of whether the OS pre-splits the line or lumps it in one
> argument, but '#!/bin/env -S bash -' is also unlikely to work portably
> as 'env -S' is not yet in widespread usage.
> 
> > > 
> > > I intend to contribute a shell script to virt-p2v; do I need to use the hyphen in the shebang? (If so, I prefer the double-hyphen.)
> > 
> > Eric should be able to give the definitive answer here.
> 
> POSIX intentionally does not standardize #!, so the definitive answer
> is that there isn't one ;) But unless we plan on catering to someone
> sym- or hard-linking an alternative name for your script that begins
> with '-' (which I find overkill - no one sane does that), I think
> omitting the '-' in the #! line is the best course of action, because
> the only thing it does is add a layer of protection on non-Linux
> machines against someone creating a stupid alternative name for the
> script, which is unlikely to happen.  And omitting the '-' frees us up
> to insert /bin/env if we are worried about /bin/bash being
> non-portable.

virt-p2v is a relatively Linux-specific project.  The ISO always runs
Linux, and arguably therefore you'd always want to run the scripts
that prepare the appliance like virt-p2v-make-<foo> on Linux.

For other projects we need to worry about what FreeBSD, OpenBSD and
nowadays macOS do.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW