Libguestfs - Aug 2022 - shebang usage in bash scripts

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
--> hello world

That is, when we run the script under the name "-v" (with it being on
the PATH) and with the single command line argument "world", we'd
naively asume that the shebang translated to:

  /bin/bash -v world

where "-v" came from the script's name, and "world"
would be taken as the name of a shell script to execute. Thus, the idea would be
to prevent this (i.e., to pass options to the interpreter by renaming or
(sym)linking the shell script).

But that's not what we actually see; what we see is consistent with the
command:

/bin/bash -- -v world
--> hello world

In fact, if I append "sleep 1000" to the script, I can also fetch:

$ hexdump -C /proc/10975/cmdline
00000000  62 61 73 68 00 2d 2d 00  2d 76 00 77 6f 72 6c 64  |bash.--.-v.world|
00000010  00                                                |.|
00000011

So glibc and/or the Linux kernel already inserts the "--"
option/operand delimiter!

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.)

Thanks!
Laszlo

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
> --> hello world
> 
> That is, when we run the script under the name "-v" (with it
being on the PATH) and with the single command line argument "world",
we'd naively asume that the shebang translated to:
> 
>   /bin/bash -v world
> 
> where "-v" came from the script's name, and "world"
would be taken as the name of a shell script to execute. Thus, the idea would be
to prevent this (i.e., to pass options to the interpreter by renaming or
(sym)linking the shell script).
> 
> But that's not what we actually see; what we see is consistent with the
command:
> 
> /bin/bash -- -v world
> --> hello world
> 
> In fact, if I append "sleep 1000" to the script, I can also
fetch:
> 
> $ hexdump -C /proc/10975/cmdline
> 00000000  62 61 73 68 00 2d 2d 00  2d 76 00 77 6f 72 6c 64 
|bash.--.-v.world|
> 00000010  00                                                |.|
> 00000011
> 
> So glibc and/or the Linux kernel already inserts the "--"
option/operand delimiter!
> 
> 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.

Rich.

-- 
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