/ 
Shell-Cmd-2.10
River stage zero No dependents
/ Shell::Cmd

NAME

Shell::Cmd - run shell commands with enhanced support

SYNOPSIS

use Shell::Cmd;

$obj = new Shell::Cmd;

DESCRIPTION

A very common use of perl is to act as a wrapper around shell commands where perl is used to prepare the shell commands, execute them, and deal with the resulting output. Even where the bulk of the work is actually done in the perl script, creating small shell scripts within it to do some portion of the task is common.

In the simplest form, running shell commands can be done very simply using the system() call, backticks, or several other ways, but I usually find myself wanting to do a bit (and sometimes a lot) more, especially when I am writing a long-term script that I want to be robust. In these cases, I frequently ended up writing a subroutine to run the shell command(s) for me with added functionality.

This module is designed to take a list of shell commands and automatically turn them into a shell script (using only basic shell commands) which adds some common desirable functionality including:

Handle STDOUT/STDERR

Commonly, I want to treat STDOUT and STDERR in some way. I may want to keep one or both of them, or discard one or both of them, or merge them.

Command echoing

A common option I want to set is command echoing where the commands I run are echoed as they are run. I want to be able to easily turn this on or off.

Dry-run

Another common option is to create a dry-run environment where the shell commands may be printed, but not actually run. Again, I want to be able to turn this on and off easily.

Error trapping

Even though I may combine a number of shell commands into a single script (so that it all runs in one shell), I still want to have built in error trapping at a per-command basis. I want to take a series of commands and know exactly which one failed.

I also want to be able to control what happens to commands that are listed after a failed command. I may want to ignore an error and continue to run the remaining commands. I may want to simply exit. Or I may want to echo, but not run the remaining commands so that I can see what didn't get completed.

Shell environment

I sometimes want to set up some environment for the script such as what directory it will be run in and what environment variables should be set in advance.

Command alternates

Sometimes, especially if you are running the script on multiple platforms, you may not know which command you should use. You can of course generate a platform specific script, but an alternative is to specify alternate commands. If ANY of those commands succeed, then that portion of the script succeeds.

Command retrying

Occasionally you have a command that may fail, but on retrying, it will succeed. This is especially true when some effect from a previous command takes some amount of time to actually go into effect. By allowing a certain number of retries, you can often work around this situation.

Remote execution

Sometimes you want to run the commands locally. Other times, you want to run it remotely using ssh. When running remotely, you may want to run the same script on multiple hosts.

SSH handling

When running on multiple hosts using SSH, sometimes you need to run the script serially (i.e. one host at a time), but other times, it would be nice to run it in parallel to speed up execution. When running in parallel, you should be able specify how many instances to run at at time.

Quoting and special characters

Since shell commands often have quotes, dollar signs, and other special characters, this module can handle that for you by properly escaping them as necessary.

This module is designed to run multiple commands in a single shell, wrapping them in very simple, standard shell commands to automatically add all of this functionality.

METHODS

new
$obj = new Shell::Cmd;

This creates a new object containing commands.

version
$vers = $obj->version();

Returns the version of this module.

cmd
$err=$obj->cmd($cmd [,\%options], $cmd [,\%options], ...);

This is used to add one or more commands to the list of commands that will be executed.

Here, each $cmd is a string containing a command, or a listref where each element in the list is a command.

In the listref form, the list of commands are alternates to try until one succeeds, and the command only fails if all of the alternates fail. This might be used to specify different paths to an executable, or different executables that perform essentially the same function, but which might not all be available on all platforms.

For example, if you wanted to run a command to get the contents of a web site, and you didn't know which of curl, wget, or lftp were available, you might use something like this:

$err = $obj->cmd([ "wget $URL", "curl $URL", "lftp $URL"]);

and in this case, it would try wget, and if that failed, it would try curl, and if that failed, it would try lftp. The command will only fail if all three alternates fail.

Each command (or list of alternates) can have options passed in. These options apply only to this command (or list), and are described in the "PER-COMMAND OPTIONS" section below.

All of the commands stored in $obj will be run in a single shell, so it is fine to gather information in one command and store it in a shell variable for use in a later command. Also, it is not necessary (or desirable) to include a trailing semi-colon on the command as these will be added automatically as needed.

An error is returned if any of the arguments are invalid.

It should be noted that no attempt is made to see if the syntax of the shell command is correct. That is beyond the scope of this module.

If only simple lists of commands are used, handling them is relatively straightforward, but trying to include commands that affect the flow of the script (such as while...done), if...else, and the like) then handling can be much more complicated. Only a subset of these are supported. Refer to the "FLOW COMMANDS" section below.

run
$obj->run();

This prepares a shell script based on the commands and options entered and runs it as appropriate. There are several different ways in which the commands can be run, and these are described in the options method below. The most important option is the mode option which sets the mode which determines the form of the script, and how it is run.

If $obj is in 'run' mode, the method can be called as either:

($err)    = $obj->run();
$err      = $obj->run();

In this mode, the script is run, and output is sent directly to STDOUT and STDERR as appropriate for the options specified. In essence, this generates a script and runs it with the system() call.

If a temporary script file is used, but it cannot be written, $err will be -2. Otherwise, the return value of the method is the exit code of the script (which is the exit code of the first failing command).

If $obj is in 'dry-run' mode, the method should be called as either:

($script) = $obj->run();
$script   = $obj->run();

In this mode, the commands are not actually executed. Instead, the script is built and returned. The form of the script is determined by the script option described below.

If $obj is in 'script' mode, the output from the commands are kept for further analysis. The method should be called as:

@out      = $obj->run();

Here, @out is a list of command output descriptors:

@out      = ( FAILED_COMMAND, COMMAND_SUMMARY_1, COMMAND_SUMMARY_2, ... )

Here, FAILED_COMMAND is the command number that failed. If FAILED_COMMAND is 0, it means that the script completed successfully. If it is -1, it means that the script initialization failed before any of the commands were run. This typically means that an invalid directory was specified with the dire method. If FAILED_COMMAND is any other value, it will be an integer I. In this case, the failed command is described in COMMAND_SUMMARY_I.

For each command that ran, the output is returned along with information about the command status. The format of each COMMAND_SUMMARY is a listref of the form:

COMMAND_SUMMARY = [ CMD_NUM, STATUS, ALT_SUMMARY_1, ALT_SUMMARY_2, ... ]

CMD_NUM is the number of the command as it was entered with the cmd method starting at 0. Each ALT_SUMMARY is the summary for one of the alternates for this command.

STATUS is one of the strings:

''       : Any command that succeeds will be given either a
succ       STATUS of an empty string, or the string 'succ'.
           If the B<failure> option is set to 'continue',
           all commands which succeed are given the empty
           string until a command fails.  Commands run after
           that are given a STATUS of 'succ'.

           If B<failure> is set to any other option, only
           the empty string is ever assigned as STATUS for
           a successful command.

retried  : If a command is retried, all attempts may fail
           (except the final one) and it is not counted as
           a script failure.  These failing commands are
           given this STATUS.

exit     : The first command that actually fails is given
fail       the status 'exit'.  Other commands that fail
           (if the B<failure> option is set to 'continue')
           are given the status 'fail'.

disp     : This is only available if the B<failure>
           option is set to 'display', and will apply to
           all commands after the failing command.

It should be noted that if a command is retried multiple times, each attemp will be returned as a separate COMMAND_SUMMARY structure. Since any attempt but the final one failed, the status in those will all be 'retried'. Only the final attempt will include a status indicating success or failure for the command as a whole.

Each alternate will return a separate summary, also a listref, of the form:

ALT_SUMMARY_i = [ CMD, EXIT, STDOUT, STDERR ]

CMD is the full command that was run.

EXIT is the exit value from that command. Note that if a command has alternates, then one or more of them may fail (causing an exit value to appear here) without causing the overall command to fail.

STDOUT and STDERR are the output produced by that command. Depending on the values of the output and f-output options, one or both of these may be empty, even if the command actually produced that type of output. So, if the value of output is 'stderr', the the STDOUT will be empty, even if the command produced something. Both are listrefs of lines.

ssh
$obj->ssh(@hosts);

This behaves similar to the run method except it will run the commands on each host in @hosts using ssh.

In 'dry-run' mode, the call is:

@script = $obj->ssh(@hosts);

In 'run' mode, the call is:

@err = $obj->ssh(@hosts);

In 'script' mode, the call is:

($out1,$out2,...) = $obj->ssh(@hosts);

where $out1 is a reference to the list of output on the 1st host, etc.

Note that when running in parallel in 'run' mode, the output that is printed to the terminal will be a mix of the output from each of the hosts the commands are being run on.

If the tmp_script option is set but the file cannot be written, the return value in both cases will be (-2). If the file cannot be copied to a host, then the return value for that host will be -3.

flush
$obj->flush( [@opts] );

If @opts is not given, it removes the commands, directory, and environment stored in the object, and resets the options to the default values.

If @opts is given, it can include any of the following:

commands   : clears commands
dire       : clears the directory
env        : clears the environment
opts       : clears all options
dire
$err = $obj->dire($dire);

This method is used to set the dire option. For a description, please see the entry in "GLOBAL OPTIONS" below.

You can also check the value that is set using:

$dire = $obj->dire();
mode
$err = $obj->mode($mode);

This method is used to set the mode option. For a description, please see the entry in "GLOBAL OPTIONS" below.

You can also check the value that is set using:

$mode = $obj->mode();
env
$obj->env(VAR, VAL, VAR, VAL, ...);

This can be called any number of times to set some environment variables. If $val is undef, the environment variable will be explicitly unset.

You can also query the environment variables with:

@env = $obj->env();

It will return a list of ( VAR VAL VAR VAL ... ).

options
$err = $obj->options(%options);

This can be used to set some options about what will be done when the commands are run.

The hash is of the form:

%options = ( OPTION => VALUE,
             OPTION => VALUE, ...)

The options are defined in the "GLOBAL OPTIONS" section below.

GLOBAL OPTIONS

The following global options exist can can be set using the options method:

mode

The mode option determines how the commands will be handled by the run method. The following values are available.

dry-run
run
script

The default option is to use 'run' mode.

The 'dry-run' mode will simply return the script that would have been run, but it doesn't actually run it.

The 'run' mode is the standard way to run commands in an interactive setting. It will run the commands in real-time and allow you to watch STDOUT and/or STDERR (depending on the options you choose) as they run.

The 'script' mode is more appropriate for running in an unattended script. It gathers the output and post-processes it allowing for more useful handling of the output. For example, you could discard the output from commands that succeed and keep only the output for the one that failed.

The mode option can also be set using the mode method.

tmp_script, tmp_script_keep

The tmp_script option is used to specify a temporary script name.

The script that is generated by this module may exceed the length of a string that can be passed directly to a shell. In order to avoid this problem, the script can be stored in a temporary script file (set with the tmp_script option) which will be executed.

Once execution is complete, the temporary script file will be removed unless the tmp_script_keep option is set.

ssh_script, ssh_script_keep

These are related to the tmp_script and tmp_script_keep options. If tmp_script is created, then when the ssh method is used to run the script remotely, it is copied to the remote host (via. scp) to a temporary location (given by ssh_script). The remote script is then removed (unless ssh_script_keep is passed in).

If tmp_script is set but ssh_script is NOT, ssh_script defaults to the same value as tmp_script.

ssh_script_keep defaults to 0, even if tmp_script_keep is set.

dire

The dire option is use to specify the directory where all of the the commands should be run. This can be overridden on a per-command basis using the cmd method, but all commands not specifically set will run in this directory.

This does NOT check the existence of the directory until the commands are actually run since the commands may be run via. ssh.

The dire option can also be set using the dire method.

output

The output option can be one of the following:

both
merged
stdout
stderr
quiet

In the 'run' mode, these determine what output will be displayed.

It can display only STDOUT, only STDERR, or both, or both can be discarded with the 'quiet' option. The default is to include 'both'. The 'merged' option is used to display both but merge STDERR into STDOUT (using a 2>&1 redirection).

The default in both cases is 'both'.

In the 'script' mode, the output is determined by using a combination of the 'output' and 'f-output' options as described next.

f-output

The f-output option can be one of the following:

both
merged
stdout
stderr
quiet

and is only used in 'script' mode.

In 'script' mode, all of the output is gathered and analyzed after the commands are run, so the output can be tailored to whether a command completed successfully or failed.

The 'output' option controls what output is returned for commands that completed successfully, and the 'f-output' option controls the output given for a failed command. Note that if a command has multiple alternatives, and one of them succeeds, the command is treated as succeeding, and the output for all alternates (even the individual ones that fail) will follow the rules of 'output' rather than 'f-output'.

It should be noted that since the command runs fully before it can be seen whether it failed or not, STDOUT/STDERR will either be separate or merged based on the first option. If the first option is 'both', 'stdout, or 'stderr', they will be separate, and the 'f-merged' option is not supported (it will be replaced by 'f-both'). If the first option is 'merged', then the 'f-both', 'f-stdout', and 'f-stderr' options are not supported and will be replaced by 'f-merged'.

The default to these are 'both' and 'f-both'.

script

The script option is used only in dry-run mode.

When commands are run in dry-run mode, a script is produced. The form of that script is controlled by this option. The value may be any of:

run
script
simple

If the value is 'run' (which is the default), the script produced will be exactly the script produced in the 'run' mode. If the value is 'script', the script will be the script produced in 'script' mode.

If the value is 'simple', the script will simply be the list of commands with the minimum necessary additions to handle directory and environment variables. No additional scripting will be added to do error checking or add other functionality.

echo

The 'echo' option is used in 'run' mode. With it, you can choose whether or not the commands should be displayed before they are run.

The values are:

echo
noecho
failed

With 'echo' and 'noecho', commands will be displayed or NOT displayed respectively.

If the value is 'failed', a command that failed will be displayed. Since it has already run, the command will be echoed after execution rather than before.

failure

When a command fails, there are several alternatives that can be done. Values for this option are:

exit
display
continue

The default is 'exit'. With this option, the shell will stop executing commands once one has failed. The exit code is the code from the failed command. When running in script mode, no information about remaining commands is returned.

With the 'display' option, the simple script is displayed (in 'run' mode) and the point of failure is marked. The exit code is the code from the failed command.

With the 'continue' option, remaining command are executed, but the overall exit values is still set to point at the first failed command.

ssh_num

When running a command on multiple hosts via SSH, it is possible to run them serially (one at a time) or in parallel.

This option can be set to a number 0 or more. If the number is 1, then only a single ssh connection will be made at a time so the hosts will all be contacted serially.

If the option is set to 0, all of the hosts will be run simultaneously.

If the option is set to N, N simultaneous connections will be allowed.

ssh_sleep

When running a command on multiple hosts via SSH, it is sometimes desirable to stagger them slightly so multiple copies are running at the same time, but not at EXACTLY the same time.

If this option is set to 0 (the default), all of the commands will be run with no delay. If it is set to the value N, commands will sleep a random amount of time (from 0 to N seconds) before running.

ssh:XXX

When running a command on a remote host via. ssh, the Net::OpenSSH module is used.

Every option that can be passed to the 'new' method can be set here. For example, if you want to call Net::OpenSSH as:

$ssh = Net::OpenSSH->new($host, user => $user_name);

just set the option:

ssh:user = $user_name

PER-COMMAND OPTIONS

The following options exists that can be applied to individual commands. They can be set in the cmd method.

dire

The dire option refers to the directory which this single command should be executed in. The value of the option is the directory.

This will basically wrap a command in:

CURR_DIR=`pwd`
cd $dire
COMMAND
cd $CURR_DIR
noredir

If the noredir option is included, no command line redirection is done for this command. Most commands automatically redirect STDOUT and STDERR based on the output and f-output global options.

If the command explicitly sends these to somewhere (such as a log file or temporary file), use the noredir option so automatic redirection is not done.

retry, sleep

The retry and sleep options can be used to retry a command.

Sometimes, a command may fail but running it a second time can succeed. Often, a command completes, but for various reasons, it takes a certain amount of time after the command completes for the full results to take effect. A later command might be run before those results have taken effect, but rerunning it a few seconds later would succeed.

With the retry option, you can retry a command. The value of the retry option should be an integer (N). If N is greater than 1, the command will be run up to N times total. Any other value of N will be ignored, and the command will run only a single time.

There can be an optional sleep time between running the command. The optional sleep option (which should also be an integer) sets the number of seconds between retries. If the value is 0, or not an integer, there will be no delay between retries.

This command will be marked as failed only if all of the retries fail.

You cannot retry a flow command.

check

Sometimes, a command is written such that the exit code does not accurately reflect whether the command failed or not. It may produce a zero exit code but still have failed, or it may have succeeded but still produce an error code.

In these cases, you can supply a command with this option which will check the result of the command and set the error flag appropriately.

If the command succeeded, the error flag should be set to zero. If it failed, it should be set to something non-zero.

If this is given for a command which has alternatives, it will be run after every alternative.

FLOW COMMANDS

When simple shell commands are given, there is no ambiguity about how to treat each, so handling them is relatively simple. Simple commands are fully supported, and all of the functionality described above can be added.

However, when flow commands are included, there is a great deal of difficulty in handling them properly. This is because the added functionality is done by wrapping each command with some enclosing shell structure to add it. It is very easy to wrap regular shell commands in some enclosing structure. For example, it is easy to turn:

mycommand arg1 arg2

into

if [ SOME_CONDITION ]; then
   DO_SOMETHING
   mycommand arg1 arg2
   DO_SOMETHING
fi

but you can't wrap the commands of an if/else/fi block or a while/done block. As a result, each type of flow must be considered carefully.

Currently, the only flow command which is supported is the if...elsif...else...fi structure. Limited loop support for while/done, until/done and for/done is planned, but is not yet available.

In order to recognize them, the commands will be partiall parsed, and they must be of the forms:

if ... ; then
elif ... ; then
else
fi

where '...' may be any string. In other words, the first line must start with if, followed by whitespace, and end with a ';' followed by optional whitespace, followed by 'then'.

The alternate formatting of:

if ...
then
fi

is not supported.

KNOWN PROBLEMS

Minimal support for complex scripts

These methods work best for simple lists of commands. Using if...then...else is allowed, but other flow commands (such as while...done, etc.) will not work in most instances.

LICENSE

This script is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

AUTHOR

Sullivan Beck (sbeck@cpan.org)