I need to get an example file file from a find command in a Perl script to create another system call afterwards. For some reason, the find command gets stuck when I call it from the script. Here is what I need to do:
my $search_dir = "/something/like/this/??/??/??";
# the triple '??' are needed here
my $cmd = "find $search_dir -name \"\*.$var1.token1.$var2.ext\" | head -n 1";
my $first_example_file = `$cmd`; chomp $first_example_file;
This gets stuck when I run it through Perl, it never finishes executing the command, whereas the constructed $cmd runs in no time if I copy+paste it and run in in my bash terminal. Any ideas?
Try using the File::Find perl module for finding files. If you would like to use bash's find in your perl then you might have to use $(..) in your command.
I am not in to perl … just trying to help out.
Update:
As stated in the comments by Rohaq you can also use File::Find::Rule
I'd wager globbing (shell metacharacter expansion) is involved. But regardless, try and chop the command up. Does it work without the pipe? What about without the ?? in the pathname? What happens if you prepend 'echo' ("echo find ...")? Still hanging? Then you can try it under perl -d - the debugger; perldoc perldebug is your friend.
Related
What would be an example of how I can call a shell command, say 'ls -a' in a Perl script and the way to retrieve the output of the command as well?
How to run a shell script from a Perl program
1. Using system system($command, #arguments);
For example:
system("sh", "script.sh", "--help" );
system("sh script.sh --help");
System will execute the $command with
#arguments and return to your script when finished. You may check $!
for certain errors passed to the OS by the external application. Read
the documentation for system for the nuances of how various
invocations are slightly different.
2. Using exec
This is very similar to the use of system, but it will
terminate your script upon execution. Again, read the documentation
for exec for more.
3. Using backticks or qx//
my $output = `script.sh --option`;
my $output = qx/script.sh --option/;
The backtick operator and it's equivalent qx//, excute the command and options inside the operator and return that commands output to STDOUT when it finishes.
There are also ways to run external applications through creative use of open, but this is advanced use; read the documentation for more.
From Perl HowTo, the most common ways to execute external commands from Perl are:
my $files = `ls -la` — captures the output of the command in $files
system "touch ~/foo" — if you don't want to capture the command's output
exec "vim ~/foo" — if you don't want to return to the script after executing the command
open(my $file, '|-', "grep foo"); print $file "foo\nbar" — if you want to pipe input into the command
Examples
`ls -l`;
system("ls -l");
exec("ls -l");
Look at the open function in Perl - especially the variants using a '|' (pipe) in the arguments. Done correctly, you'll get a file handle that you can use to read the output of the command. The back tick operators also do this.
You might also want to review whether Perl has access to the C functions that the command itself uses. For example, for ls -a, you could use the opendir function, and then read the file names with the readdir function, and finally close the directory with (surprise) the closedir function. This has a number of benefits - precision probably being more important than speed. Using these functions, you can get the correct data even if the file names contain odd characters like newline.
As you become more experienced with using Perl, you'll find that there are fewer and fewer occasions when you need to run shell commands. For example, one way to get a list of files is to use Perl's built-in glob function. If you want the list in sorted order you could combine it with the built-in sort function. If you want details about each file, you can use the stat function. Here's an example:
#!/usr/bin/perl
use strict;
use warnings;
foreach my $file ( sort glob('/home/grant/*') ) {
my($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,$atime,$mtime,$ctime,$blksize,$blocks)
= stat($file);
printf("%-40s %8u bytes\n", $file, $size);
}
There are a lot of ways you can call a shell command from a Perl script, such as:
back tick
ls which captures the output and gives back to you.
system
system('ls');
open
Refer #17 here: Perl programming tips
You might want to look into open2 and open3 in case you need bidirectional communication.
I have been using system and qq to run linux programs inside perl. And it has worked well.
#!/usr/bin/perl # A hashbang line in perl
use strict; # It can save you a lot of time and headache
use warnings; # It helps you find typing mistakes
# my keyword in Perl declares the listed variable
my $adduser = '/usr/sbin/adduser';
my $edquota = '/usr/sbin/edquota';
my $chage = '/usr/bin/chage';
my $quota = '/usr/bin/quota';
my $nomeinteiro;
my $username;
my $home;
# system() function executes a system shell command
# qq() can be used in place of double quotes
system qq($adduser --home $home --gecos "$fullname" $username);
system qq($edquota -p john $username);
system qq($chage -E \$(date -d +180days +%Y-%m-%d) $username);
system qq($chage -l $username);
system qq($quota -s $username);
I have a perl script which takes 2 arguments as follows and calls appropriate function depending on the argument. I call this script from bash, but i want to call it from perl, is it possible?
/opt/sbin/script.pl --group="value1" --rule="value2";
Also the script exits with a return value that I would like to read.
The Perl equivalent of sh command
/opt/sbin/script.pl --group="value1" --rule="value2"
is
system('/opt/sbin/script.pl', '--group=value1', '--rule=value2');
You could also launch the command in a shell by using the following, though I'd avoid doing so:
system(q{/opt/sbin/script.pl --group="value1" --rule="value2"});
Just like you'd have to do in sh, you'll have to follow up with some error checking (whichever approach you took). You can do so by using use autodie qw( system );. Check the docs for how to do it "manually" if you want more flexibility.
If you want to capture the output:
$foo = `/opt/sbin/script.pl --group="value1" --rule="value2"`;
If you want to capture the exit status, but send script.pl's output to stdout:
$status = system "/opt/sbin/script.pl --group=value1 --rule=value2";
If you want to read its output like from a file:
open SCRIPT, "/opt/sbin/script.pl --group=value1 --rule=value2 |" or die $!;
while (<SCRIPT>) ...
Yes. You can use system, exec, or <backticks>.
The main difference between system and exec is that exec "executes a command and never returns.
Example of system:
system("perl", "foo.pl", "arg");
Im trying to write a very simple script that copies files from several remote computers to my computer. I've written the following in perl:
#Comps = ("comp1","comp2","comp3","comp4");
foreach $comp (#Comps)
{
system("copy /Y \\\\$comp\\c\$\\myfolder\\$comp*.log C:\\Program Files\\My Cont Folder\\input\\");
}
I get a syntax error and cant understand why.
Thanks!
Given the case that Perl is passing the characters to the shell, the only way that the shell will process "C:\Program Files" as one argument is to put double quotes around it. (Or sometimes just with one double quote preceding it.)
So you might need a few more backslashes (so I qq-ed it):
system( qq{cmd /c copy /Y \\\\$comp\\c\$\\myfolder\\$comp*.log "C:\\Program Files\\My Cont Folder\\input\\"} )
Looks like you've got error in the command itself, because the perl code ran fine. (Gave me 4 times "command not found" since I'm on Unix).
I would suggest using list form of system() call: system("copy", "/Y", $src, $dst); and, probably, even sprintf() to fill out $src and $dst.
Of course, you can always check what exactly gets executed by running print before system. I often hesitate and often print.
Perl and windows paths don't mix very well, why not use File::Copy instead?
use File::Copy;
copy($file1,$file2) or die "Copy failed: $!";
And then you can use forward slash in your paths instead. Much easier.
What would be an example of how I can call a shell command, say 'ls -a' in a Perl script and the way to retrieve the output of the command as well?
How to run a shell script from a Perl program
1. Using system system($command, #arguments);
For example:
system("sh", "script.sh", "--help" );
system("sh script.sh --help");
System will execute the $command with
#arguments and return to your script when finished. You may check $!
for certain errors passed to the OS by the external application. Read
the documentation for system for the nuances of how various
invocations are slightly different.
2. Using exec
This is very similar to the use of system, but it will
terminate your script upon execution. Again, read the documentation
for exec for more.
3. Using backticks or qx//
my $output = `script.sh --option`;
my $output = qx/script.sh --option/;
The backtick operator and it's equivalent qx//, excute the command and options inside the operator and return that commands output to STDOUT when it finishes.
There are also ways to run external applications through creative use of open, but this is advanced use; read the documentation for more.
From Perl HowTo, the most common ways to execute external commands from Perl are:
my $files = `ls -la` — captures the output of the command in $files
system "touch ~/foo" — if you don't want to capture the command's output
exec "vim ~/foo" — if you don't want to return to the script after executing the command
open(my $file, '|-', "grep foo"); print $file "foo\nbar" — if you want to pipe input into the command
Examples
`ls -l`;
system("ls -l");
exec("ls -l");
Look at the open function in Perl - especially the variants using a '|' (pipe) in the arguments. Done correctly, you'll get a file handle that you can use to read the output of the command. The back tick operators also do this.
You might also want to review whether Perl has access to the C functions that the command itself uses. For example, for ls -a, you could use the opendir function, and then read the file names with the readdir function, and finally close the directory with (surprise) the closedir function. This has a number of benefits - precision probably being more important than speed. Using these functions, you can get the correct data even if the file names contain odd characters like newline.
As you become more experienced with using Perl, you'll find that there are fewer and fewer occasions when you need to run shell commands. For example, one way to get a list of files is to use Perl's built-in glob function. If you want the list in sorted order you could combine it with the built-in sort function. If you want details about each file, you can use the stat function. Here's an example:
#!/usr/bin/perl
use strict;
use warnings;
foreach my $file ( sort glob('/home/grant/*') ) {
my($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,$atime,$mtime,$ctime,$blksize,$blocks)
= stat($file);
printf("%-40s %8u bytes\n", $file, $size);
}
There are a lot of ways you can call a shell command from a Perl script, such as:
back tick
ls which captures the output and gives back to you.
system
system('ls');
open
Refer #17 here: Perl programming tips
You might want to look into open2 and open3 in case you need bidirectional communication.
I have been using system and qq to run linux programs inside perl. And it has worked well.
#!/usr/bin/perl # A hashbang line in perl
use strict; # It can save you a lot of time and headache
use warnings; # It helps you find typing mistakes
# my keyword in Perl declares the listed variable
my $adduser = '/usr/sbin/adduser';
my $edquota = '/usr/sbin/edquota';
my $chage = '/usr/bin/chage';
my $quota = '/usr/bin/quota';
my $nomeinteiro;
my $username;
my $home;
# system() function executes a system shell command
# qq() can be used in place of double quotes
system qq($adduser --home $home --gecos "$fullname" $username);
system qq($edquota -p john $username);
system qq($chage -E \$(date -d +180days +%Y-%m-%d) $username);
system qq($chage -l $username);
system qq($quota -s $username);
As I understand (Perl is new to me) Perl can be used to script against a Unix command line. What I want to do is run (hardcoded) command line calls, and search the output of these calls for RegEx matches. Is there a way to do this simply in Perl? How?
EDIT: Sequence here is:
-Call another program.
-Run a regex against its output.
my $command = "ls -l /";
my #output = `$command`;
for (#output) {
print if /^d/;
}
The qx// quasi-quoting operator (for which backticks are a shortcut) is stolen from shell syntax: run the string as a command in a new shell, and return its output (as a string or a list, depending on context). See perlop for details.
You can also open a pipe:
open my $pipe, "$command |";
while (<$pipe>) {
# do stuff
}
close $pipe;
This allows you to (a) avoid gathering the entire command's output into memory at once, and (b) gives you finer control over running the command. For example, you can avoid having the command be parsed by the shell:
open my $pipe, '-|', #command, '< single argument not mangled by shell >';
See perlipc for more details on that.
You might be able to get away without Perl, as others have mentioned. However, if there is some Perl feature you need, such as extended regex features or additional text manipulation, you can pipe your output to perl then do what you need. Perl's -e switch let's you specify the Perl program on the command line:
command | perl -ne 'print if /.../'
There are several other switches you can pass to perl to make it very powerful on the command line. These are documented in perlrun. Also check out some of the articles in Randal Schwartz's Unix Review column, especially his first article for them. You can also google for Perl one liners to find lots of examples.
Do you need Perl at all? How about
command -I use | grep "myregexp" && dosomething
right in the shell?
#!/usr/bin/perl
sub my_action() {
print "Implement some action here\n";
}
open PROG, "/path/to/your/command|" or die $!;
while (<PROG>) {
/your_regexp_here/ and my_action();
print $_;
}
close PROG;
This will scan output from your command, match regexps and do some action (which now is printing the line)
In Perl you can use backticks to execute commands on the shell. Here is a document on using backticks. I'm not sure about how to capture the output, but I'm sure there's more than a way to do it.
You indeed use a one-liner in a case like this. I recently coded up one that I use, among other ways, to produce output which lists the directory structure present in a .zip archive (one dir entry per line). So using that output as an example of command output that we'd like to filter, we could put a pipe in and then use perl with the -n -e flags to filter the incoming data (and/or do other things with it):
[command_producing_text_output] | perl -MFile::Path -n -e \
"BEGIN{#PTM=()} if (m{^perl/(bin|lib(?!/site))}) {chomp;push #PTM,$_}" ^
-e "END{#WDD=mkpath (\#PTM,1);" ^
-e "printf qq/Created %u dirs to reflect part of structure present in the .ZIP file\n/, scalar(#WDD);}"
the shell syntax used, including: quoting of perl code and escaping of newlines, reflects CMD.exe usage in Windows NT-like consoles. If you need to, mentally replace
"^" with "\" and " with ' in the appropriate places.
The one-liner above adds only the directory names that start with "perl/bin" or
"perl/lib (not followed by "/site"); it then creates those directories. You wind
up with a (empty) tree that you can use for whatever evil purposes you desire.
The main point is to illustrate that there are flags available (-n, -p) to
allow perl to loop over each input record (line), and that what you can do is unlimited in terms of complexity.