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 _______________________________________________ Libguestfs mailing list Libguestfs@redhat.com https://listman.redhat.com/mailman/listinfo/libguestfs
