FYI: #!"shebang!" – the powerful secret sauce of command-line scripting

sundialsvcs · 05-04-2022, 08:56 AM

Ahh, yes ... the "BASH script." A simple scripting language that was built into a popular Unix/Linux shell that was never really intended to have one. (The only shell that I know of which was 'intended to have one' was Dr. Korn's shell, ksh, which implements a very impressive (for its time ...) built-in language that very few people today actually use.)

A disturbing number of scripts still use "bash scripting," and a disturbing number of computer programmers still think that they are limited to this. They are not. Shells provide a far more powerful feature: colloquially known as "shebang." Written as: #!.

If the first line of your script begins with – for example – #!/usr/bin/perl – then the shell will silently pass control over to [Perl] to handle the remainder of the script. And the user will never know.

"Perl ... PHP ... Ruby ... pick your oyster."

(Strictly speaking, any executable file can be used here.)

This concept was further enhanced by the env command, which is specifically intended to be used on a "shebang" line to add further goodness to it ... such as, among other things, "automatically discovering where [Perl] is on this machine." It also takes care of a few other things that the shell by itself doesn't do as well.

I now refer you to man env (and, man bash) for further details.

So ... keep this in mind. Most Linux distros routinely install several of these language interpreters, and most of a vendor's built-in commands and scripts use one or more of them. Therefore, you do not have to resort to "computer calisthenics" to create a perfectly-functional "command-line command." You are not limited to bash's built-in but very limited facilities. Full-featured programming environments, which were designed to be so, are at your beck and call when writing any command-line script.

(Incidentally, this is exactly the reason why many programming languages today use the # character in column-one to denote a comment line.)

Turbocapitalist · 05-04-2022, 11:54 AM

Quote:

Originally Posted by sundialsvcs

"Perl ... PHP ... Ruby ... pick your oyster."

(Strictly speaking, any executable file can be used here.)

Some more which I have made use of are GNUplot, AWK, and R

Code:

$ grep -h -m 1 -E '^#' * | sort -u

#!/usr/bin/awk -f
#!/usr/bin/gnuplot
#!/usr/bin/Rscript

As you note, the locations can vary, especially with *BSD. But for the most part on GNU/Linux distros generally keep the scripting languages in the expected place.

rknichols · 05-04-2022, 01:36 PM

Quote:

Originally Posted by sundialsvcs

Shells provide a far more powerful feature: colloquially known as "shebang." Written as: #!.

That is actually a kernel feature. The shell isn't involved at all unless, of course, the specified interpreter happens to be a shell.

michaelk · 05-04-2022, 03:29 PM

Exactly, when the script is set as executable the program loader parses and executes the specified program i.e bash, php, python etc.

The shebang (#!) is actually a magic number. Magic numbers are basically what tells the system, applications whatever the type of file versus its extension like Windows.

sundialsvcs · 05-04-2022, 06:37 PM

Quote:

Originally Posted by michaelk

The shebang (#!) is actually a magic number. Magic numbers are basically what tells the system, applications whatever the type of file versus its extension like Windows.

Yes, and the file command can do that interpretation for you.

(Although, as is to be expected, Windows actually does not rely upon the file-extension alone. All files have predictable binary headers.)