I need to write a perl script that calls a c-shell script that calls yet another perl script. I cannot change the c-shell script or the perl script it calls. One of the args that needs to be passed is a quotes string with spaces. If I use backticks to call the c-shell, and I run the c-shell with tcsh, the quoted string is respected as a single entity. However, if I run the c-shell with source, it is not.
I feel that I need to use 'source' because when the c-shell is called by users from the command line, it is called through an alias that sources the c-shell. E.g.
alias top "source top.csh"
Consider these...
topmost.pl
#!/usr/bin/env perl
use strict;
print "Try with tcsh...\n";
my $msg = `tcsh ./top.csh -arg1 "this line has spaces"`;
print "$msg\n";
print "Try with source...\n";
my $msg = `source ./top.csh -arg1 "this line has spaces"`;
print "$msg\n";
exit;
top.csh is simply....
perl ./subperl.pl $*:q
exit
And subperl.pl is...
#!/usr/bin/env perl
use strict;
print "In subperl.pl\n";
foreach $x (#ARGV) {
print "$x\n";
}
print "The End\n";
exit;
When I run the topmost.pl script, I get...
Try with tcsh...
In subperl.pl
-arg1
this line has spaces
The End
Try with source...
In subperl.pl
-arg1
this
line
has
spaces:q
The End
Why does the "sourced" call to the top.csh script fail to respect the quotes ?
#Kaz has the answer as to why your code isn't working. This answer is about how to avoid this class of problems entirely.
First, if you can, add a #!/bin/tcsh to top.csh and make it executable (ie. chmod +x). Now it can be executed as top.csh without needing to know what shell to use.
Then you'll want to avoid using `` for anything but very simple commands. This is because `` is interpreted by the shell and now you need to worry about shell special characters and escapes and spaces... it's a mess. What you need is a way to call external programs without invoking a shell.
You can do this by passing a list to system, but system cannot capture the output.
system "tcsh", "./top.csh", "-arg1", "this line has spaces";
While you can cobble something together with open and pipes, it's better to use a pre-existing library such as IPC::System::Simple.
use IPC::System::Simple qw(capturex);
# Or capturex("./top.csh", ...) if you add a #! to top.csh.
my $msg = capturex("tcsh", "./top.csh", "-arg1", "this line has spaces");
For more involved interactions with executables, look into System::Command or IPC::Run.
Needless to say, Perl scripts which call shell scripts which call Perl scripts is a bit of a nightmare to maintain. Rather than do that, it is better to scoop the guts of subperl.pl out into a Perl library and have both subperl.pl and your code use that library.
The command in backticks is being interpreted by your system interpreter (invoked via /bin/sh), which I'm guessing might be GNU Bash. Or, in any case, it seems to be some shell which understands the source command, and almost certainly a POSIX-like shell. That source command quite probably tells that shell to read a script written in that shell's own language. So, for instance, if that shell happens to be Bash, it will treat that as a Bash script1, not as a Tcsh script.
The only way both could work is if the script is a "polyglot": a program which can be interpreted by either tcsh or the system shell that is used by perl to implement backticks.
(An easy example of a C Shell + POSIX shell polyglot is a script that contains nothing but a sequence of trivial commands consisting of space separated words like cp from to.)
Your script isn't a polyglot; only Tcsh understands the :q syntax, not the other shell.
More precisely, if /bin/sh is Bash, the original source ... command as well as the contents of the sourced top.csh script will be treated as a POSIX-mode Bash script, since when Bash is invoked as /bin/sh, it turns off its POSIX-incompatible behaviors. So even if Bash's pathname expansion supported the Tcsh :q mechanism, it would almost certainly be turned off under POSIX mode because $*:q already has a firm meaning in POSIX.
Related
Hi I have a Unix Shell Script call Food.sh ; I have a Perl Script call Track.pl. Is there a way where I can put Track.pl's code in to Food.sh code and still have it work ? Track.pl requires one arugement to label a name of a folder.
Basically it will run like this.
Unix Shell Script codes RUN
step into
Perl Script codes RUN
Put in name of folder for Perl script
Rest of script runs
exit out.
You have a few options.
You can pass the code to Perl using -e/-E.
...
perl -e'
print "Hello, World!\n";
'
...
Con: Escaping can be a pain.[1]
You can pass the code via STDIN.
...
perl <<'END_OF_PERL'
print "Hello, World!\n";
END_OF_PERL
...
Con: The script can't use STDIN.
You can create a virtual file.
...
perl <(
cat <<'END_OF_PERL'
print "Hello, World!\n";
END_OF_PERL
)
...
Con: Wordy.
You can take advantage of perl's -x option.
...
perl -x -- "$0"
...
exit
#!perl
print "Hello, World!\n";
Con: Can only have one snippet.
$0 is the path to the shell script being executed. It's passed to perl as the program to run. The -x tells Perl to start executing at the #!perl line rather than the first line.
Ref: perlrun
Instances of ' in the program needs to escaped using '\''.
You could also rewrite the program to avoid using '. For example, you could use double-quoted string literals instead of single-quoted string literals. Or replace the delimiter of single-quoted string literals (e.g. q{...} instead of '...'). As for single-quoted inside of double-quoted and regex literals, these can be replaced with \x27, which you might find nicer than '\''.
(I'm assuming your goal is just to have all of the code in a single file so that you don't have multiple files to install)
Sure, there's a way to do this, but it's cumbersome. You might want to consider converting the shell script entirely to Perl (or the Perl script entirely to shell).
So ... A way to do this might be:
#!/bin/sh
echo "shell"
perl -E '
say "perl with arg=$ARGV[0]"
' fred
echo "shell again"
Of course, you'd have to be careful with your quotes within the Perl part of the program.
You might also be able to use a heredoc for the Perl part to avoid quoting issues, but I'm not sure about that.
I came across the following example. I tried to google but could not find much so I'm posting this question here.
What is the benefit of executing the perl script like this?
How can we make the shell script work like a "normal" shell script once we are through executing the perl code?
Here's the code:
#!/bin/ksh
#! -*- perl -*-
eval 'exec $PERLLOCATION/bin/perl -x $0 ${1+"$#"} ;'
if 0;
print "hello world\n";
# how can I make it behave like a "normal" shell script from this point onwards? What needs to be done?
# echo "hello world" ### this results in error
This idiom is described in the perlrun documentation.
The -x switch scans the whole file and ignores anything that appears before the first line that begins with #! and also contains the word perl.
It means that your system will run the script with the Perl interpreter whether you invoke the script with perl or with a shell command (sh/bash/ksh/etc.)
That is,
$ perl this_script
and
$ sh this_script
will both run the script with perl.
To address your second question, this idiom has just about nothing to do with combining shell script and Perl script in the same file. There are a few different ways to approach that problem, but maybe the most readable way is to write in shell script, but use the shell's heredoc notation to invoke perl code.
#!/bin/bash
# this is a bash script, but there is some Perl in here too
echo this line is printed from the shell
echo now let\'s run some Perl
perl <<EOF
# this is now perl script until we get to the EOF
print "This line is printed from Perl\n";
EOF
echo now this is from the shell script again
1. If you start a Perl script in the usual way:
#!/usr/bin/perl
print "hello world\n";
the #! line will only work if the Perl interpreter is actually installed under /usr/bin. The perl/ksh bilingual script you show is a tricky kluge to make the script work even if perl is installed somewhere else. For more information, see e.g. this.
2. You can't. When the shell process encounters the exec command, it terminates and hands control over to perl. (Technically, it executes perl in place of the shell, without creating a new process.) The only way to run more shell commands after that would be to launch a new shell.
It's way simpler than what's already been posted.
#!$PERLLOCATION/bin/perl
doesn't work because the shebang (#!) line is interpreted by the kernel (not the shell), and the kernel doesn't do variable interpolation.
The code invokes ksh to expand the environment variable and to launch the specified installation of Perl.
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 written a library in Perl that contains a certain function, that returns information about a server as a character string. Can I call this function from a shell directly?
My boss asks "Can you call it from a shell directly for the time being?" Because he said that, I think I should be able to do it, but how do I do it?
perl -MServerlib=server_information -e 'print server_information()'
Is another way to do this, but only if Serverlib exports server_information sub. If it doesn't, you would need to do the below instead:
perl -MServerlib -e 'print MServerlib::server_information()'
As perl's command line arguments are a bit inscrutable, I'd wrap it in a simpler perl script that calls the function. For example, create a script serverinfo which contains:
#!/usr/bin/perl
require 'library.pl';
say library::getServerInformation();
then run:
chmod u+x serverinfo
The advantage of doing it this way is the output and arguments of the script can be corrected if the function itself changes. A command line script like this can be thought of as an API, which shouldn't change when the implementation changes.