System::Command supports the following methods:
new
my $cmd = System::Command->new( @cmd )
Runs an external command using the list in @cmd.
If @cmd contains a hash reference, it is taken as an option hash.
If several option hashes are passed to "new()", they will be merged together with individual values being
overridden by those (with the same key) from hashes that appear later in the list.
To allow subclasses to support their own set of options, unrecognized options are silently ignored.
The recognized keys are:
"cwd"
The currentworkingdirectory in which the command will be run.
"env"
A hashref containing key / values to add to the command environment.
If several option hashes define the "env" key, the hashes they point to will be merged into one
(instead of the last one taking precedence).
If a value is "undef", the variable corresponding to the key will be removed from the environment.
"input"
A string that is send to the command's standard input, which is then closed.
Using the empty string as "input" will close the command's standard input without writing to it.
Using "undef" as "input" will not do anything. This behaviour provides a way to modify previous
options populated by some other part of the program.
On some systems, some commands may close standard input on startup, which will cause a SIGPIPE when
trying to write to it. This will raise an exception.
"interactive"
If true, the command will actually be run using the "system" in perlfunc builtin. If "STDIN" is not a
terminal, the constructor will die.
Not reaper object will be created, and the "stdin", "stdout" and "stderr" filehandles will point to
dummy closed handles. The "exit", "signal" and "core" attributes will be correctly set.
(Added in version 1.114.)
"setpgrp"
By default, the spawned process is made the leader of its own process group using "setpgrp( 0, 0 )"
(if possible). This enables sending a signal to the command and all its child processes at once:
# negative signal is sent to the process group
kill -SIGKILL, $cmd->pid;
Setting the "setpgrp" option to a false value disables this behaviour.
(Added in version 1.110.)
"trace"
The "trace" option defines the trace settings for System::Command. The "SYSTEM_COMMAND_TRACE"
environment variable can be used to specify a global trace setting at startup. The environment
variable overrides individual "trace" options.
If "trace" or "SYSTEM_COMMAND_TRACE" contains an "=" character then what follows it is used as the
name of the file to append the trace to. When using the "trace" option, it is recommended to use an
absolute path for the trace file, in case the main program "chdir()" before calling System::Command.
At trace level 1, only the command line is shown:
System::Command cmd[12834]: /usr/bin/git commit -m "Test option hash in new()"
Note: Command-line parameters containing whitespace will be properly quoted.
At trace level 2, the options values are shown:
System::Command opt[12834]: cwd => "/tmp/kHkPUBIVWd"
System::Command opt[12834]: fatal => {128 => 1,129 => 1}
System::Command opt[12834]: git => "/usr/bin/git"
Note: The "fatal" and "git" options in the example above are actually used by Git::Repository to
determine the command to be run, and ignored by System::Command. References are dumped using
Data::Dumper.
At trace level 3, the content of the "env" option is also listed:
System::Command env[12834]: GIT_AUTHOR_EMAIL => "author\@example.com"
System::Command env[12834]: GIT_AUTHOR_NAME => "Example author"
If the command cannot be spawned, the trace will show "!" instead of the pid:
System::Command cmd[!]: does-not-exist
(Added in version 1.108.)
exit
core
signal
The above three options can be set to point to a reference to a scalar, which will be automatically
updated when the command is terminated. See the "Accessors" section for details about what the
attributes of the same name mean.
(Added in version 1.114.)
The System::Command object returned by "new()" has a number of attributes defined (see below).
close
$cmd->close;
Close all pipes to the child process, collects exit status, etc. and defines a number of attributes (see
below).
Returns the invocant, so one can do things like:
my $exit = $cmd->close->exit;
is_terminated
if ( $cmd->is_terminated ) {...}
Returns a true value if the underlying process was terminated.
If the process was indeed terminated, collects exit status, etc. and defines the same attributes as
"close()", but does not close all pipes to the child process.
spawn
my ( $pid, $in, $out, $err ) = System::Command->spawn(@cmd);
This shortcut method calls "new()" (and so accepts options in the same manner) and directly returns the
"pid", "stdin", "stdout" and "stderr" attributes, in that order.
(Added in version 1.01.)
loop_on
$cmd->loop_on(
stdout => sub { ... },
stderr => sub { ... },
);
This method calls the corresponding code references with each line produced on the standard output and
errput of the command.
If the "stdout" or "stderr" argument is not given, the default is to silently drop the data for "stdout",
and to pass through (to STDERR) the data for "stderr". To prevent any processing, pass a false value to
the parameter.
For example, the following line will silently run the command to completion:
$cmd->loop_on( stderr => '' );
The method blocks until the command is completed (or rather, until its output and errput handles have
been closed), or until one of the callbacks returns a false value.
Data is read using readline, which depends on $/ for its definition of a "line". To that effect, the
method takes a third optional argument, "input_record_separator", which sets the value for $/ for the
duration of the call.
CaveatEmptor: since "loop_on" is line-based, it may block if either output or errput sends incomplete
lines (e.g. if the command is some sort of interactive shell with a prompt).
The return value is true if the command exited with status 0, and false otherwise (i.e. the Unix
traditional definition of success).
(Added in version 1.117.)
Accessors
The attributes of a System::Command object are also accessible through a number of accessors.
The object returned by "new()" will have the following attributes defined:
cmdline
Return the command-line actually executed, as a list of strings.
options
The merged list of options used to run the command.
pid The PID of the underlying command.
stdin
A filehandle opened in write mode to the child process' standard input.
stdout
A filehandle opened in read mode to the child process' standard output.
stderr
A filehandle opened in read mode to the child process' standard error output.
Regarding the handles to the child process, note that in the following code:
my $fh = System::Command->new( @cmd )->stdout;
$fh is opened and points to the output handle of the child process, while the anonymous System::Command
object has been destroyed. Once $fh is destroyed, the subprocess will be reaped, thus avoiding zombies.
(System::Command::Reaper undertakes this process.)
After the call to "close()" or after "is_terminated()" returns true, the following attributes will be
defined (note that the accessors always run "is_terminated()", to improve their chance of getting a value
if the process just finished):
exit
The exit status of the underlying command.
signal
The signal, if any, that killed the command.
core
A boolean value indicating if the command dumped core.
Even when not having a reference to the System::Command object any more, it's still possible to get the
"exit", "core" or "signal" values, using the options of the same name:
my $fh = System::Command->new( @cmd, { exit => \my $exit } )->stdout;
Once the command is terminated, the $exit variable will contain the value that would have been returned
by the "exit()" method.
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