I am working on transcribing an outdated file from perl to python and got caught up with some perl syntax.
my $jobID = `echo \$JOBID`;
chomp($jobID);
unless ($jobID) {
print "Please specify a job\n";
exit;
}
Thus far, I have been able to transcribe all of the command-line parsing extremely easily but am very fairly stuck here with what exactly this code is doing (specifically the echo within the declaration on line 1).
Within the perl script cmd-line parsing options - that enables one to set the jobID - it states that "default = $JOBID". So my assumption is that the first line in this code simply sets this default jobID until one is specified.
If this is the case why would you need to use echo within the variable default declaration? Is this good practice in perl or am I missing a larger picture?
I have tried searching low and high for this but can't seem to google ninja my way to anything useful.
Any help on the 'echo' would be greatly appreciated (or any good reads on this as well)!
This is one way to get a value from a shell variable. The backticks (`) run the shell command and give you the output. So the echo is running inside of a shell and in this case it just returns the one shell variable. A cleaner way to get this in Perl is to use %ENV like so:
my $jobID = $ENV{'JOBID'};
This also removes the need for chomp, avoids creating an extra process, and is much more efficient.
It is evaluating an environment variable named $JOBID and storing the result in $jobID, which (as duskwuff points out) is better accomplished using $ENV{JOBID}.
The backticks around the echo \$JOBID tell Perl to invoke the specified command in a subshell and return the output of the invoked command.
Related
I am writing a large Perl script, which needs to utilize other existing Perl scripts. The problem is the main script needs to reference many different scripts from different folders. For example the main script would be contained in:
/perl/programs/io
It may need to run a script which is stored in:
/perl/programs/tools
Note that there are other orthogonal folders besides tools so I need to be able to access any of them on the fly.
Currently this is what I got:
my $mynumber = '../tools/convert.pl bin2dec 1011';
In theory it should move back from the io directory then enter the appropriate tool directory and call the convert.pl script while passing it the parameters.
All this does is store the string in the single quotes to $myNumber.
I like to assign the output of a command to an array so I can loop through the array to find error or other messages. For example if I'm making a zip file to email to someone I want to check to see if the zip program had any errors before I continue to make and send the email.
#msgs = `zip -f myfile.zip *.pl`; # Use backticks
You can also assign the output to a scalar:
$msg = `ls -al *.pl`; # Use backticks
To run any system command or script all you have to do is use `backticks`. From observing another programer's perl code, I misread these strange quotes for 'single quotes'.
backticks are also nice because they return the text in STDOUT to your perl script so that the output can be assigned to a variable, something I have found impossible if using system("");
The similar question answer does not work with my version of perl. The line
use IPC::System::Simple qw(system capture);
throws some errors. However just using system works, like this:
my $mynumber = system($^X, "../tools/convert.pl", 'bin2dec', '1011');
I can use the above without setting equal to something to execute scripts which return no value and are only sent arguments.
This seems to be the easiest way to do what I need to and the entire programs folder can be moved anywhere and it will still work as no parent directories above programs are used.
I thought this must be simple, but I haven't found a viable solution.
The problem is as simple as this: I want to execute a system command and capture the output in Perl variable. The command is specified in Perl array (containing command and parameters, e.g. #cmd = ('mycmd', '-opt1', 'arg1', 'val1')).
I don't want to use forking, i.e. open(FROM_KID, '-|') is not an option. I know that if I had the command in a string I can achieve this with backticks. So perhaps this problem reduces to converting #cmd array into a string. In my case, the command arguments can contain spaces.
Is there a simple way to convert #cmd array into string that can be used with backticks, but such that all arguments are properly quoted? Also ideally without using any external libraries.
Thanks!!
You may be looking for String::ShellQuote. Note that only Bourne shell quoting is supported.
But backticks also perform an implicit fork, and if they in any way differ from the implicit fork of pipe open, I for one never noticed. :-\
So, I'm writing a Perl script which at one point needs to process the output of another script. I tried to do this by calling the script in backticks:
my #output = `scriptName`;
I have tested the script I want to call in backticks, and it works just fine--in the same shell I'm calling my script in, even. But when I call it as part of the script, it produces no output. The variable is left empty.
I've tried executing the command with system(), but there is still no output. I have no idea why. However, the specific arguments I'm passing into the script have caused me problems before until I fixed my PATH variable. Does calling a script through a Perl script result in different environment variables somehow?
Edit: Okay, here's a potential issue. I tried using backticks and system() to print out my PATH variable, and both of them are coming up blank. Is my Perl script unable to use my PATH for some reason?
The most common cause of problems such as these, is difference in relative paths. I have a tendency of using absolute paths for that reason.
In addition, it sounds to me like your subscript might be printing to STDERR and not STDOUT. Backticks only capture STDOUT, so you need to do a redirect with the help of 2>&1
my #output = `scriptName 2>&1`;
NB: The redirect doesn't work with all shells (I believe it was tcsh that didn't support it back when i had a similar problem). Bash takes the redirect just fine.
Environment isn't your problem, unless you're explicitly adjusting it within perl, prior to calling your script. My suggestion would be - double check permissions on your script, and check relative paths. I note you don't have ./scriptName - so if it's not in your path, perl won't be able to find it either.
Am pretty much aware of passing variables between shell scripts using 'EXPORT' command. But am stuck with passing a variable value from a perl script to shell script in UNIX operating systems.
Let me explain it clearly.
I have a parent shell called parent_shell.sh. Inside this shell script am using a variable called 'file_name' which I should fetch from child perl script.
So inside my parent_shell.sh script it will be like as follows,
perl my_perl_script.pl
file_name = 'variable' #from perl above perl script
Hope this is clear. Please let me know if it is not clear.
Thanks
Modifying the global %ENV hash in perl is equivalent to exporting variables in shell.
However, exporting variables in environment, in any language, only affects child processes, period. You can't modify parent process environment in any way.
The child script can only return anything by printing on standard output and standard error and by it's status, but that is a number 0-127 (well, it's a number 0-255, but shell can only reliably process the values up to 127).
If you just need one value from the perl script, simply print the value and use process substitution from the shell:
file_name=$(perl my_perl_script.pl)
if it is more, you can print a shell code to set the variables and use shell's eval, but make sure you quote the values correctly before printing from perl.
How to debug a perl script used inside another perlscript. We can debug both separately. Is it possible to debug in one step?
Ex:
!/user/bin/perl
my $param= 8;
my #res=\`perl extract.pl $config`;
print "The results is .... #res\n";
Similarly, can we debug the perl scripts used inside shell script?
Thanks.
You can add -d switch to the shebang line of the script you are invoking:
#!/usr/bin/perl -d
A dynamic solution:
my $dswitch = $INC{"perl5db.pl"} ? "-d" : "";
my #res=`perl $dswitch extract.pl $config`;
$INC{"perl5db.pl"} is true when you are using the debugger, and false when you are not.
The "correct" thing to do would be to change your design so you are not calling the other script through a system command. Then you would have a single debugging environment.
The best design would be to make extract.pl into a module.
You could also use the do command to import the code from the other file: do 'extract.pl'. Then the only change needed would probably be the way you pass $config in.
Either of these approaches would be quite simple to implement and would have other benefits for continued development. Maybe you have a good reason not to do it this way, but I would consider this first if you haven't already.
Just debug extract.pl passing in the value in $config. If you don't know what it is, the age-old printf will do that for you.
say "\$config='$config'";
And then do:
perl -d extract.pl 'Stuff that printed in config'.
As the two are isolated processes, the only communication is what is passed on the command line and what is printed by the called script. So the called script should be capable of being tested without the calling script. (Unless you've set up some sort of IPC or shared memory or something tricky like that.)