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I need to run a shell command within a perl script(It needs to be a shell command) but I need to use a variable in a perl loop
Ex. doing something like
#!/usr/bin/perl
for my $i (0..9) {
`echo $i`;
}
should produce the output
0
1
2
3
4
5
6
7
8
9
You can do it in several ways, depending on what exactly is needed. Generally, this involves use of system, backticks (with operator form qx), or open. There is also a number of great modules for it.
The qx returns its STDOUT to the program. If you need it assign it to a variable.
for my $i (0..9) {
my $cmd = "external-command $i";
my $cmdout = qx($cmd); # needs error checking ($?)
print $cmdout; # process as needed
}
The output can also be assigned to an array, when each line becomes one array element. Depending on what your command is you may have to pay close attention to how to quote it correctly, and String::ShellQuote can help a lot. If you want to get the command's STDERR as well use
my $cmd = "external-command $i 2>&1";
See qx in perlop (or readpipe), which also discusses error checking; also see $? in pervar.
If you don't need the command's output in your script, or specifically want it to wind up together with your script's other output, use system. Commands run by it inherit the script's STDOUT and STDERR so what they print to standard streams will go where your script's output goes (unless it is redirected by the command invocation).
for my $i (0..9) {
my $cmd = "external-command $i";
my $ret = system($cmd);
if ($ret != 0) {
warn "Error running $cmd: $ret";
# interrogate further, see documentation for 'system'
}
}
The error handling above is rudimentary, see docs for more. Finally, there are various forms of open that run a process. For example, see it in perlfaq8, along with a full review of what you are asking.
And then there are modules, that make this kind of work far easier and better. From simple to more capable, some of well-known ones are IPC::System::Simple, Capture::Tiny, IPC::Run3, and IPC::Run. Also see links (examples) assembled in this post, for example.
A command can be executed via a shell or not (but via execvp) depending on how exactly it is invoked, as discussed in the docs. The phrase "shell command" strictly refers to a facility provided by the shell itself, in which case you of course want to run it that way. However, it is also often used for a program that we normally execute by typing in a terminal, so it gets executed by the shell. In this case you may (or may not) want to bypass the shell.
Also note that the above all block (do not return control before completing), so the script continues only after they're done. This seems to be what you want.
If you just want to pass through the shell output, simply do
print STDOUT `echo $i`;
I have a problem with the system function. I want to store the system functions output to a variable.
For example,
system("ls");
Here I want all the file names in the current directory to store in a variable. I know that I can do this by redirecting the output into a file and read from that and store that to a variable. But I want a efficient way than that. Is there any way .
No, you cannot store the values of the ls output ,
since system always execute the command as a child process ,
so try with backtick `command` which executes the command in the current
process itself!
The easiest way uses backticks or qx():
my $value = qx(ls);
print $value;
The output is similar to the ls.
My answer does not address your problem. However, if you REALLY want to do directory listing, don't call system ls like that. Use opendir(), readdir(), or a while loop.
For example,
while (<*>){
print $_ ."\n";
}
In fact, if it's not a third-party proprietary program, always try to user Perl's own functions.
As abubacker stated, you can use backticks to capture the output of a program into a variable for later use. However, if you also need to check for exceptional return values, or bypass invoking the shell, it's time to bring in a CPAN module, IPC::System::Simple:
use IPC::System::Simple qw(capture);
# Capture output into $result and throw exception on failure
my $result = capture("some_command");
This module can be called in a variety of ways, and allows you to customize which error return values are "acceptable", whether to bypass the shell or not, and how to handle grouping of arguments. It also provides a drop-in replacement for system() which adds more error-checking.
The official Perl documentation for the built-in system function states:
This is not what you want to use to
capture the output from a command, for
that you should use merely backticks
or qx//, as described in "STRING" in
perlop.
There are numerous ways to easily access the docs:
At the command line: perldoc -f
system
Online at perldoc.perl.org.
Search the web using google.
If you want each directory listing stored into a separate array element, use:
my #entries = qx(ls);
Use backticks to store output in a variable
$output = `ls`;
A quick and simple way to do this is to use qx() specifically for your example:
my $output = qx(ls 2>&1);
The 2>&1 part is to capture both stdout and stderr.
Since it has not been mentioned by other answers yet, you can also use Capture::Tiny to store any arbitrary STDOUT (and/or STDERR) into a variable, including from the system command.
use strict;
use warnings;
use Capture::Tiny 'capture_stdout';
my ($stdout, $return) = capture_stdout { system 'ls' };
# error checking for system return value required here
In the middle of a Perl script, there is a system command I want to execute. I have a string that contains the data that needs to be fed into stdin (the command only accepts input from stdin), and I need to capture the output written to stdout. I've looked at the various methods of executing system commands in Perl, and the open function seems to be what I need, except that it looks like I can only capture stdin or stdout, not both.
At the moment, it seems like my best solution is to use open, redirect stdout into a temporary file, and read from the file after the command finishes. Is there a better solution?
IPC::Open2/3 are fine, but I've found that usually all I really need is IPC::Run3, which handles the simple cases really well with minimal complexity:
use IPC::Run3; # Exports run3() by default
run3( \#cmd, \$in, \$out, \$err );
The documentation compares IPC::Run3 to other alternatives. It's worth a read even if you don't decide to use it.
The perlipc documentation covers many ways that you can do this, including IPC::Open2 and IPC::Open3.
Somewhere at the top of your script, include the line
use IPC::Open2;
That will include the necessary module, usually installed with most Perl distributions by default. (If you don't have it, you could install it using CPAN.) Then, instead of open, call:
$pid = open2($cmd_out, $cmd_in, 'some cmd and args');
You can send data to your command by sending it to $cmd_in and then read your command's output by reading from $cmd_out.
If you also want to be able to read the command's stderr stream, you can use the IPC::Open3 module instead.
IPC::Open3 would probably do what you want. It can capture STDERR and STDOUT.
http://metacpan.org/pod/IPC::Open3
A very easy way to do this that I recently found is the IPC::Filter module. It lets you do the job extremely intuitively:
$output = filter $input, 'somecmd', '--with', 'various=args', '--etc';
Note how it invokes your command without going through the shell if you pass it a list. It also does a reasonable job of handling errors for common utilities. (On failure, it dies, using the text from STDERR as its error message; on success, STDERR is just discarded.)
Of course, it’s not suitable for huge amounts of data since it provides no way of doing any streaming processing; also, the error handling might not be granular enough for your needs. But it makes the many simple cases really really simple.
I think you want to take a look at IPC::Open2
There is a special perl command for it
open2()
More info can be found on: http://sunsite.ualberta.ca/Documentation/Misc/perl-5.6.1/lib/IPC/Open2.html
If you do not want to include extra packages, you can just do
open(TMP,">tmpfile");
print TMP $tmpdata ;
open(RES,"$yourcommand|");
$res = "" ;
while(<RES>){
$res .= $_ ;
}
which is the contrary of what you suggested, but should work also.
I always do it this way if I'm only expecting a single line of output or want to split the result on something other than a newline:
my $result = qx( command args 2>&1 );
my $rc=$?;
# $rc >> 8 is the exit code of the called program.
if ($rc != 0 ) {
error();
}
If you want to deal with a multi-line response, get the result as an array:
my #lines = qx( command args 2>&1 );
foreach ( my $line ) (#lines) {
if ( $line =~ /some pattern/ ) {
do_something();
}
}
I've got a Perl script that needs to execute another Perl script. This second script can be executed directly on the command line, but I need to execute it from within my first program. I'll need to pass it a few parameters that would normally be passed in when it's run standalone (the first script runs periodically, and executes the second script under a certain set of system conditions).
Preliminary Google searches suggest using backticks or a system() call. Are there any other ways to run it? (I'm guessing yes, since it's Perl we're talking about :P ) Which method is preferred if I need to capture output from the invoked program (and, if possible, pipe that output as it executes to stdout as though the second program were invoked directly)?
(Edit: oh, now SO suggests some related questions. This one is close, but not exactly the same as what I'm asking. The second program will likely take an hour or more to run (lots of I/O), so I'm not sure a one-off invocation is the right fit for this.)
You can just do it.
{
local #ARGV = qw<param1 param2 param3>;
do '/home/buddy/myscript.pl';
}
Prevents the overhead of loading in another copy of perl.
The location of your current perl interpreter can be found in the special variable $^X. This is important if perl is not in your path, or if you have multiple perl versions available but which to make sure you're using the same one across the board.
When executing external commands, including other Perl programs, determining if they actually ran can be quite difficult. Inspecting $? can leave lasting mental scars, so I prefer to use IPC::System::Simple (available from the CPAN):
use strict;
use warnings;
use IPC::System::Simple qw(system capture);
# Run a command, wait until it finishes, and make sure it works.
# Output from this program goes directly to STDOUT, and it can take input
# from your STDIN if required.
system($^X, "yourscript.pl", #ARGS);
# Run a command, wait until it finishes, and make sure it works.
# The output of this command is captured into $results.
my $results = capture($^X, "yourscript.pl", #ARGS);
In both of the above examples any arguments you wish to pass to your external program go into #ARGS. The shell is also avoided in both of the above examples, which gives you a small speed advantage, and avoids any unwanted interactions involving shell meta-characters. The above code also expects your second program to return a zero exit value to indicate success; if that's not the case, you can specify an additional first argument of allowable exit values:
# Both of these commands allow an exit value of 0, 1 or 2 to be considered
# a successful execution of the command.
system( [0,1,2], $^X, "yourscript.pl", #ARGS );
# OR
capture( [0,1,2, $^X, "yourscript.pl", #ARGS );
If you have a long-running process and you want to process its data while it's being generated, then you're probably going to need a piped open, or one of the more heavyweight IPC modules from the CPAN.
Having said all that, any time you need to be calling another Perl program from Perl, you may wish to consider if using a module would be a better choice. Starting another program carries quite a few overheads, both in terms of start-up costs, and I/O costs for moving data between processes. It also significantly increases the difficulty of error handling. If you can turn your external program into a module, you may find it simplifies your overall design.
All the best,
Paul
I can think of a few ways to do this. You already mentioned the first two, so I won't go into detail on them.
backticks: $retVal = `perl somePerlScript.pl`;
system() call
eval
The eval can be accomplished by slurping the other file into a string (or a list of strings), then 'eval'ing the strings. Heres a sample:
#!/usr/bin/perl
open PERLFILE, "<somePerlScript.pl";
undef $/; # this allows me to slurp the file, ignoring newlines
my $program = <PERLFILE>;
eval $program;
4 . do: do 'somePerlScript.pl'
You already got good answers to your question, but there's always the posibility to take a different point of view: maybe you should consider refactoring the script that you want to run from the first script. Turn the functionality into a module. Use the module from the first and from the second script.
If you need to asynchronously call your external script -you just want to launch it and not wait for it to finish-, then :
# On Unix systems, either of these will execute and just carry-on
# You can't collect output that way
`myscript.pl &`;
system ('myscript.pl &');
# On Windows systems the equivalent would be
`start myscript.pl`;
system ('start myscript.pl');
# If you just want to execute another script and terminate the current one
exec ('myscript.pl');
Use backticks if you need to capture the output of the command.
Use system if you do not need to capture the output of the command.
TMTOWTDI: so there are other ways too, but those are the two easiest and most likely.
See the perlipc documentation for several options for interprocess communication.
If your first script merely sets up the environment for the second script, you may be looking for exec.
#!/usr/bin/perl
use strict;
open(OUTPUT, "date|") or die "Failed to create process: $!\n";
while (<OUTPUT>)
{
print;
}
close(OUTPUT);
print "Process exited with value " . ($? >> 8) . "\n";
This will start the process date and pipe the output of the command to the OUTPUT filehandle which you can process a line at a time. When the command is finished you can close the output filehandle and retrieve the return value of the process. Replace date with whatever you want.
I wanted to do something like this to offload non-subroutines into an external file to make editing easier. I actually made this into a subroutine. The advantage of this way is that those "my" variables in the external file get declared in the main namespace. If you use 'do' they apparently don't migrate to the main namespace. Note the presentation below doesn't include error handling
sub getcode($) {
my #list;
my $filename = shift;
open (INFILE, "< $filename");
#list = <INFILE>;
close (INFILE);
return \#list;
}
# and to use it:
my $codelist = [];
$codelist = getcode('sourcefile.pl');
eval join ("", #$codelist);
Many beginning programmers write code like this:
sub copy_file ($$) {
my $from = shift;
my $to = shift;
`cp $from $to`;
}
Is this bad, and why? Should backticks ever be used? If so, how?
A few people have already mentioned that you should only use backticks when:
You need to capture (or supress) the output.
There exists no built-in function or Perl module to do the same task, or you have a good reason not to use the module or built-in.
You sanitise your input.
You check the return value.
Unfortunately, things like checking the return value properly can be quite challenging. Did it die to a signal? Did it run to completion, but return a funny exit status? The standard ways of trying to interpret $? are just awful.
I'd recommend using the IPC::System::Simple module's capture() and system() functions rather than backticks. The capture() function works just like backticks, except that:
It provides detailed diagnostics if the command doesn't start, is killed by a signal, or returns an unexpected exit value.
It provides detailed diagnostics if passed tainted data.
It provides an easy mechanism for specifying acceptable exit values.
It allows you to call backticks without the shell, if you want to.
It provides reliable mechanisms for avoiding the shell, even if you use a single argument.
The commands also work consistently across operating systems and Perl versions, unlike Perl's built-in system() which may not check for tainted data when called with multiple arguments on older versions of Perl (eg, 5.6.0 with multiple arguments), or which may call the shell anyway under Windows.
As an example, the following code snippet will save the results of a call to perldoc into a scalar, avoids the shell, and throws an exception if the page cannot be found (since perldoc returns 1).
#!/usr/bin/perl -w
use strict;
use IPC::System::Simple qw(capture);
# Make sure we're called with command-line arguments.
#ARGV or die "Usage: $0 arguments\n";
my $documentation = capture('perldoc', #ARGV);
IPC::System::Simple is pure Perl, works on 5.6.0 and above, and doesn't have any dependencies that wouldn't normally come with your Perl distribution. (On Windows it depends upon a Win32:: module that comes with both ActiveState and Strawberry Perl).
Disclaimer: I'm the author of IPC::System::Simple, so I may show some bias.
The rule is simple: never use backticks if you can find a built-in to do the same job, or if their is a robust module on the CPAN which will do it for you. Backticks often rely on unportable code and even if you untaint the variables, you can still open yourself up to a lot of security holes.
Never use backticks with user data unless you have very tightly specified what is allowed (not what is disallowed -- you'll miss things)! This is very, very dangerous.
Backticks should be used if and only if you need to capture the output of a command. Otherwise, system() should be used. And, of course, if there's a Perl function or CPAN module that does the job, this should be used instead of either.
In either case, two things are strongly encouraged:
First, sanitize all inputs: Use Taint mode (-T) if the code is exposed to possible untrusted input. Even if it's not, make sure to handle (or prevent) funky characters like space or the three kinds of quote.
Second, check the return code to make sure the command succeeded. Here is an example of how to do so:
my $cmd = "./do_something.sh foo bar";
my $output = `$cmd`;
if ($?) {
die "Error running [$cmd]";
}
Another way to capture stdout(in addition to pid and exit code) is to use IPC::Open3 possibily negating the use of both system and backticks.
Use backticks when you want to collect the output from the command.
Otherwise system() is a better choice, especially if you don't need to invoke a shell to handle metacharacters or command parsing. You can avoid that by passing a list to system(), eg system('cp', 'foo', 'bar') (however you'd probably do better to use a module for that particular example :))
In Perl, there's always more than one way to do anything you want. The primary point of backticks is to get the standard output of the shell command into a Perl variable. (In your example, anything that the cp command prints will be returned to the caller.) The downside of using backticks in your example is you don't check the shell command's return value; cp could fail and you wouldn't notice. You can use this with the special Perl variable $?. When I want to execute a shell command, I tend to use system:
system("cp $from $to") == 0
or die "Unable to copy $from to $to!";
(Also observe that this will fail on filenames with embedded spaces, but I presume that's not the point of the question.)
Here's a contrived example of where backticks might be useful:
my $user = `whoami`;
chomp $user;
print "Hello, $user!\n";
For more complicated cases, you can also use open as a pipe:
open WHO, "who|"
or die "who failed";
while(<WHO>) {
# Do something with each line
}
close WHO;
From the "perlop" manpage:
That doesn't mean you should go out of
your way to avoid backticks when
they're the right way to get something
done. Perl was made to be a glue
language, and one of the things it
glues together is commands. Just
understand what you're getting
yourself into.
For the case you are showing using the File::Copy module is probably best. However, to answer your question, whenever I need to run a system command I typically rely on IPC::Run3. It provides a lot of functionality such as collecting the return code and the standard and error output.
Whatever you do, as well as sanitising input and checking the return value of your code, make sure you call any external programs with their explicit, full path. e.g. say
my $user = `/bin/whoami`;
or
my $result = `/bin/cp $from $to`;
Saying just "whoami" or "cp" runs the risk of accidentally running a command other than what you intended, if the user's path changes - which is a security vulnerability that a malicious attacker could attempt to exploit.
Your example's bad because there are perl builtins to do that which are portable and usually more efficient than the backtick alternative.
They should be used only when there's no Perl builtin (or module) alternative. This is both for backticks and system() calls. Backticks are intended for capturing output of the executed command.
Backticks are only supposed to be used when you want to capture output. Using them here "looks silly." It's going to clue anyone looking at your code into the fact that you aren't very familiar with Perl.
Use backticks if you want to capture output.
Use system if you want to run a command. One advantage you'll gain is the ability to check the return status.
Use modules where possible for portability. In this case, File::Copy fits the bill.
In general, it's best to use system instead of backticks because:
system encourages the caller to check the return code of the command.
system allows "indirect object" notation, which is more secure and adds flexibility.
Backticks are culturally tied to shell scripting, which might not be common among readers of the code.
Backticks use minimal syntax for what can be a heavy command.
One reason users might be temped to use backticks instead of system is to hide STDOUT from the user. This is more easily and flexibly accomplished by redirecting the STDOUT stream:
my $cmd = 'command > /dev/null';
system($cmd) == 0 or die "system $cmd failed: $?"
Further, getting rid of STDERR is easily accomplished:
my $cmd = 'command 2> error_file.txt > /dev/null';
In situations where it makes sense to use backticks, I prefer to use the qx{} in order to emphasize that there is a heavy-weight command occurring.
On the other hand, having Another Way to Do It can really help. Sometimes you just need to see what a command prints to STDOUT. Backticks, when used as in shell scripts are just the right tool for the job.
Perl has a split personality. On the one hand it is a great scripting language that can replace the use of a shell. In this kind of one-off I-watching-the-outcome use, backticks are convenient.
When used a programming language, backticks are to be avoided. This is a lack of error
checking and, if the separate program backticks execute can be avoided, efficiency is
gained.
Aside from the above, the system function should be used when the command's output is not being used.
Backticks are for amateurs. The bullet-proof solution is a "Safe Pipe Open" (see "man perlipc"). You exec your command in another process, which allows you to first futz with STDERR, setuid, etc. Advantages: it does not rely on the shell to parse #ARGV, unlike open("$cmd $args|"), which is unreliable. You can redirect STDERR and change user priviliges without changing the behavior of your main program. This is more verbose than backticks but you can wrap it in your own function like run_cmd($cmd,#args);
sub run_cmd {
my $cmd = shift #_;
my #args = #_;
my $fh; # file handle
my $pid = open($fh, '-|');
defined($pid) or die "Could not fork";
if ($pid == 0) {
open STDERR, '>/dev/null';
# setuid() if necessary
exec ($cmd, #args) or exit 1;
}
wait; # may want to time out here?
if ($? >> 8) { die "Error running $cmd: [$?]"; }
while (<$fh>) {
# Have fun with the output of $cmd
}
close $fh;
}