Can I write a Perl program where my first line is not #!/path/?
Thank you.
The shebang (#!)is only necessary if you want to invoke the script directly at a shell prompt, e.g. ./yourscript. You can always do perl yourscript and skip the shebang.
If your concern is hard-coding a constant path (e.g. #!/usr/bin/perl as opposed to #!/usr/local/bin/perl), then use:
#!/usr/bin/env perl
This allows the Perl interpreter to be sought in your PATH, making your scripts a bit more portable (Windows aside).
Yes, from perldoc perlrun (under the -S switch):
#!/bin/sh
eval 'exec /usr/bin/perl -wS $0 ${1+"$#"}'
if $running_under_some_shell;
See that documentation for the complete story.
If you do that then you'll need to invoke Perl explicitly. If that line is there, then the system knows that it is a Perl script so when you make the script executable you can just write ./script
Related
I am new to perl scripting and I am wondering what is the use of "-w" in hashbang of perl scripts.
#!/usr/bin/perl -w
my $userid = $ENV{USER};
print "$userid\n";
I know that this part #!/usr/bin/perl is used to inform shell that it is a perl script so that no need to use perl before running the script.
But I couldn't find the exact meaning for -w and most of the scripts I saw have this option .
Kindly let me know if that has any significance as I could not find any difference.
Edit:
Below document is very helpful(Suggested by #Toto)
https://perldoc.perl.org/5.32.0/perlrun.html
Everything after the command is passed as arguments to the command being invoked.
The -w option to the perl command is to enable warnings.
I'm new to Perl and was hoping someone could tell me what this means exactly
eval 'exec ${PERLHOME}/bin/perl -S $0 ${1+"$#"}' # -*- perl -*-
if 0;
This is explained in perldoc perlrun:
-S
makes Perl use the PATH environment variable to search for the program
unless the name of the program contains path separators.
...
Typically this is used to emulate #! startup on platforms that don't
support #! . It's also convenient when debugging a script that uses
#! , and is thus normally found by the shell's $PATH search
mechanism.
This example works on many platforms that have a shell compatible with
Bourne shell:
#!/usr/bin/perl
eval 'exec /usr/bin/perl -wS $0 ${1+"$#"}'
if $running_under_some_shell;
The system ignores the first line and feeds the program to /bin/sh,
which proceeds to try to execute the Perl program as a shell script.
The shell executes the second line as a normal shell command, and thus
starts up the Perl interpreter. On some systems $0 doesn't always
contain the full pathname, so the -S tells Perl to search for the
program if necessary. After Perl locates the program, it parses the
lines and ignores them because the variable
$running_under_some_shell is never true. If the program will be
interpreted by csh, you will need to replace ${1+"$#"} with $* ,
even though that doesn't understand embedded spaces (and such) in the
argument list. To start up sh rather than csh, some systems may
have to replace the #! line with a line containing just a colon,
which will be politely ignored by Perl.
In short, it mimics shebang behavior for platforms that have shells compatible with Bash.
It's valid both as shell script and as a Perl program. It is used to run the Perl interpreter after all on systems where the shebang doesn't work, for some reason. It's rarely seen these days but used to be common in the early 1990s.
The comment is just a comment, but it has special meaning in Emacs, which will open the file in perl mode.
I just read #Zaid's response, which is better and more correct than mine as long as this code is on the first line of the script being executed, and no shebang exists. I've never seen this kind of substitute. Quite interesting, really.
The second line, if 0; is a part of the first line. You can tell since the first line lacks a ;. It would be more obvious if this was one long single line with the comment being after the semicolon.
So it's equivalent to:
if(0) {
eval 'exec ${PERLHOME}/bin/perl -S $0 ${1+"$#"}
}
In perl, 0 will be evaluated to false, and so the eval-clause will never execute. Presumably this condition(the if) was a quick way to disable the line. Perhaps the evaluation was once something real instead of an always-false.
See perl --help, perldoc -f eval and perldoc -f exec for information on the evaluation block itself.
The remaining trickyness (${1+"$#"}) I have no idea about. This isn't perl anyway; it's interpreted by whichever shell exec is launching (Correct me if I'm wrong on this!). If it's bash, I don't think it does anything at all and can be substituted with $#, which is the environment variable holding all commandline arguments (ie #ARGV in perl).
I often write one-liners on the command line like so:
perl -Magic -wlnaF'\t' -i.orig -e 'abracadabra($_) for (#F)'
In order to scriptify this, I could pass the same flags to the shebang line:
#!/usr/bin/perl -Magic -wlnaF'\t' -i.orig
abracadabra($_) for (#F);
However, there's two problems with this. First, if someone invokes the script by passing it to perl directly (as 'perl script.pl', as opposed to './script.pl'), the flags are ignored. Also, I can't use "/usr/bin/env perl" for this because apparently I can't pass arguments to perl when calling it with env, so I can't use a different perl installation.
Is there anyway to tell a script "Hey, always run as though you were invoked with -wlnaF'\t' -i.orig"?
You're incorrect about the perl script.pl version; Perl specifically looks for and parses options out of a #! line, even on non-Unix and if run as a script instead of directly.
The #! line is always examined for switches as the line is being
parsed. Thus, if you're on a machine that allows only one argument
with the #! line, or worse, doesn't even recognize the #! line, you
still can get consistent switch behavior regardless of how Perl was
invoked, even if -x was used to find the beginning of the program.
(...)
Parsing of the #! switches starts wherever perl is mentioned in the
line. The sequences "-*" and "- " are specifically ignored so that you
could, if you were so inclined, say
#!/bin/sh
#! -*-perl-*-
eval 'exec perl -x -wS $0 ${1+"$#"}'
if 0;
to let Perl see the -p switch.
Now, the above quote expects perl -x, but it works just as well if you start the script with
#! /usr/bin/env perl -*-perl -p-*-
(with enough characters to get past the 32-character limit on systems with that limit; see perldoc perlrun for details on that and the rest of what I quoted above).
I had the same problem with #!env perl -..., and env ended up being helpful:
$ env 'perl -w'
env: ‘perl -w’: No such file or directory
env: use -[v]S to pass options in shebang lines
So, just modify the shebang to #!/usr/bin/env -S perl -...
I have perl scripts starting with #!/usr/bin/perl or #!/usr/bin/env perl
First, what does the second version mean?
Second, I use Ubuntu. All the scripts are set as executables. When I try to run a script by simply invoking it's name (e.g. ./script.pl) I get : No such file or directory. when I invoke by perl ./script.pl it runs fine.
Why?
The #!/usr/bin/env perl uses the standard POSIX tool env to work around the "problem" that UNIX doesn't support relative paths in shebang lines (AFAIK). The env tool can be used to start a program (in this case perl) after modifying environment variables. In this case, no variables are modified and env then searches the PATH for Perl and runs it. Thus a script with that particular shebang line will work even when Perl is not installed in /usr/bin but in some other path (which must be in the PATH variable).
Then, you problem with ./script.pl not working: you said it has the executable bit(s) set, like with chmod +x script.pl ? But does it also start with a shebang (#!) line ? That is, the very first two bytes must be #! and it must be followed by a file path (to perl). That is necessary to tell the kernel with which program to run this script. If you have done so, is the path correct ? You want to try the #!/usr/bin/env perl variant ;-)
Using #!/usr/bin/env perl gets around the problem of perl not necessarily being in /usr/bin on every system; it's just there to make the script more portable
On a related note, for your second problem, is there a /usr/bin/perl and/or /usr/bin/env? If not, that would explain why running the scripts directly doesn't work; the shebang isn't handled if you run the script as an argument to perl
How do I run a Perl script on OS X?
You can run your Perl script by invoking the Perl interpreter and giving your file as input:
perl myprogram.pl
The easiest way to run a perl script is with the option:
perl myprogram.pl
However, you may find it more useful to add a shebang line at the top of the perl file.
#!/usr/bin/perl
print "Hello World!\n";
In order to execute this script, you need to add execute permissions to your program. Run:
chmod +x myprogram.pl
Now, in order to run your script, you can simply type:
./myprogram.pl
A good tutorial on Perl in OSX can be found here:
http://www.mactech.com/articles/mactech/Vol.18/18.09/PerlforMacOSX/index.html
A generic documentation on executing Perl code is of course perldoc perlrun.
To answer your question directly:
You can run a perl script on any Unix system by either having the code evaluated and executed from command line:
perl -e 'print "Hello World\n"';
Or you can save your Perl script to a file (customarily having .pl extension, say script1.pl and with the first line being #!/usr/bin/perl) and then you can execute it as any Unix program (after setting proper execute permissions)
/path/to/script/script1.pl
You can also execute a script from a file by running perl interpreter as the command and giving the script as a parameter (in this case execute permissions to the script are not needed):
perl /path/to/script/script1.pl
For some reason the whole directory didn't work for me but I just did
perl ~/Desktop/file.pl
(you could also use any folder that is in your user-folder after the ~/)