I have looked all over and found plenty of answers, and after much trial and error, I have come to this conclusion: I don't know.
I am running a thing in Perl:
my $command = sprintf('commandHereDoingSCPishThingsThatHasToRunInMyShell', options, options, options);
system($command);
When I run my script in my terminal window, I see the output of the command inside the sprintf() function.
I would like for the output to be re-directed to a file, and to not have to see the program executing on my terminal window.
Thanks!
Raw use of perl's system() is discouraged because it isn't trivial to detect errors and come up with a relevant error message. Consider using IPC::System::Simple. Since you want to capture the output, use its capture().
use IPC::System::Simple 'capture';
my $output = capture($command); # dies if command fails; catch this with eval if necessary
File::Slurp::write_file('somefile', $output);
Alternatively, just redirect the output to a file in your command:
use IPC::System::Simple 'system';
system("$command >somefile");
Related
I have a Perl CGI program. myperlcgi.pl
Within this program I have the following:
my $retval = system('extprogram')
extprogram has a print statement within.
The output from extprogram is being included within myperlcgi.pl
I tried adding
> output.txt 2>&1
to system call but did not change anything.
How do I prevent output form extprogram being used in myperlcgi.pl.
Surprised that stdout from system call is being used in myperlcgi.pl
The system command just doesn’t give you complete control over capturing STDOUT and STDERR of the executed command.
Use backticks or open to execute the command instead. That will capture the STDOUT of the command’s execution. If the command also outputs to STDERR, then you can append 2>&1 to redirect STDERR to STDOUT for capture in backticks, like so:
my $output = `$command 2>&1`;
If you really need the native return status of the executed command, you can get that information using $? or ${^CHILD_ERROR_NATIVE}. See perldoc perlvar for details.
Another option is to use the IPC::Open3 Perl library, but I find that method to be overkill for most situations.
I am using the following code to execute the ClearCase statement "Accept" from a perl script.
$acceptA = `accept $Component`;
After execution, inside my perl script, the value of $acceptA is blank.
The text displayed on the screen during execution of this line is : "ERROR You do not have permissions to ACCEPT this work."
How do I read this line? I thought it would return into the variable $acceptA as it does with the "cleartool checkin" command?
As i do not know ClearCase and how that accept works, i can only guess. Seeing how it is an error message, it might be written to STDERR instead of STDOUT and backticks only capture STDOUT of the command executed.
In that case, redirecting the commands STDERR to STDOUT would work. Try
$acceptA = `accept $Component 2>&1`
and see if that works in capturing the output in case of error as well.
I eventually redirected SYSERR to an output file which I could read/write.
open STDERR, ">/ellipse/el6.3.3_ry_sup/src/0/$logfile"
All the error messages that was displayed on the screen as part of the system command, went into $logfile.
I was also able to add to SYSERR with the following:
print STDERR "\nAccepting $Component";
Thanks for all the help.
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 $!;
while (<SCRIPT>) ...
Yes. You can use system, exec, or <backticks>.
The main difference between system and exec is that exec "executes a command and never returns.
Example of system:
system("perl", "foo.pl", "arg");
I need to call some shell commands from perl. Those commands take quite some time to finish so I'd like to see their output while waiting for completion.
The system function does not give me any output until it is completed.
The exec function gives output; however, it exits the perl script from that point, which is not what I wanted.
I am on Windows. Is there a way to accomplish this?
Backticks, or the qx command, run a command in a separate process and returns the output:
print `$command`;
print qx($command);
If you wish to see intermediate output, use open to create a handle to the command's output stream and read from it.
open my $cmd_fh, "$command |"; # <--- | at end means to make command
# output available to the handle
while (<$cmd_fh>) {
print "A line of output from the command is: $_";
}
close $cmd_fh;
What would be an example of how I can call a shell command, say 'ls -a' in a Perl script and the way to retrieve the output of the command as well?
How to run a shell script from a Perl program
1. Using system system($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);