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    Shell::Cmd - run shell commands with enhanced support

       use Shell::Cmd;

       $obj = new Shell::Cmd;

    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:

        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 (typically with a command line option in the calling

        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. If I execute
        the commands one at a time, I can get that information, but
        typically, I want to combine multiple commands in a single script
        but still have that ability.

        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.

           $obj = new Shell::Cmd;

        This creates a new object containing commands.

           $vers = $obj->version();

        Returns the version of this module.

           $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 either a string containing a command, or a
        listref where each element in the list is a string containing a

        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

        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. Commands should not
        include a trailing semi-colon 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

        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. Refer to
        the "FLOW COMMANDS" section below. Defining functions is NOT

           $ret = $obj->run();

        This prepares a shell script based on the commands and options
        entered and runs it as appropriate. The script is stored in a
        temporary file that can be set using the tmp_script option (refer to
        the "GLOBAL OPTIONS" section below).

        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 determines the form of the
        script, and how it is run.

        If the mode is run, the method is called as:

           $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()"

        The error code returned is described below in the "ERROR CODES"

        In dry-run mode, the method is called as:

           $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.

        In script mode, the method is called as:

           $err = $obj->run();

        In this case, the output from the commands are kept for further
        analysis. The "$obj-"output(...)> method may then be used to examine
        the resulting output.

        The error codes are described in the "ERROR CODES" section below.

    ssh This behaves similar to the "run" method except it will run the
        commands on each host in @hosts using ssh. The return values for
        each mode are identical to the return methods from the "run" method
        except that for both the run mode and script mode, the output is
        returned as a hash where the keys are the hosts and the values are
        the value for that host.

        For example, in run mode, the call would be:

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

        In dry-run mode, the call is identical to the run method, and it
        will return the script that would be run on each host.

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

        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.

           $ret = $obj->output(%options);
           @ret = $obj->output(%options);
           %ret = $obj->output(%options);

        This will return the output produced by running the commands in
        script mode depending on the options passed in.

        The %options argument is described below in the OUTPUT OPTIONS

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

        If @opts is not given, it removes all the data stored in the object,
        resetting it to a clean object. If @opts is given, you can clear
        specific parts of the object. Any of the following options can be

           commands   : clears all commands and their options
           env        : clears the environment
           opts       : clears all options
           out        : clears the output from running the command
                        in B<script> mode

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

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

           $err = $obj->options('dire',$dire);

        You can also check the value that is set using:

           $dire = $obj->dire();

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

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

           $err = $obj->options('mode',$dire);

        You can also check the value that is set using:

           $mode = $obj->mode();

           $obj->env($var1, $val1, $var2, $val2, ...);

        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();

           $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.

    The error code returned by the run or ssh methods are described in the
    following table:

       0       No error
       1-200   The number of the command that failed.
               Commands entered with the B<cmd> method
               are numbered starting at 1.  If 200 or
               less commands are entered, the error code
               will correspond to the command that
       201     If more than 200 commands are entered,
               and any of them beyond the 200th fail,
               the error code will be 201.
       252     Flow commands not correctly nested/closed
       253     If the temporary script cannot
               be copied to a remote host (for use in
               the B<ssh> method), this is returned.
       254     If a temporary script could not be
               created, this will be returned.
       255     If a global directory was specified that
               does not exist, this will be returned.

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

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

           run      (default)

        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 dry-run mode will not execute any commands. Instead, it will
        generate a script that WOULD have been run and returns it. The
        script can take several different forms, and is described in the
        script option below.

        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, or a number of other options.

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

        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 per-command options in 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.

        The output option can be one of the following:

           both     (default)

        In the run mode, these determine what output will be displayed. In
        script mode, it determines which output is stored in the object.
        Obviously, if output is not kept, it will not be available to
        examine using the "output" method.

        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 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     (default)

        If the value is run or script, the script produced will be exactly
        the script produced in those modes, including all of the wrapping
        shell structure to add the requested functionality.

        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.

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

        The values are:


        With echo and noecho values, commands will be displayed or NOT
        displayed respectively. With echo the commands will be displayed
        before they are run.

        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.

        Note that flow commands are not echoed.

        The failure option is used in run and script modes. When a command
        fails, there are several alternatives that can be done. Values for
        this option are:


        The default is exit. With this option, the script will stop
        executing commands once one has failed.

        The display option is only used in run mode. With it, if any command
        fails, a simple script will be displayed showing what commands
        failed to run.

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

    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 will be stored in a temporary script file
        (set with the tmp_script option) which will be executed. If not set,
        the default value for tmp_script will be:


        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.

        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

        If the option is set to N, N simultaneous connections will be
        allowed and additional hosts will be run only after others have

        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.
        If it is set to a negative value -N, it will sleep for exactly N

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

        Every option that can be passed to the "Net::OpenSSH::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

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

        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:

           cd $dire
           cd $CURR_DIR

        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 global option.

        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.

        Since the command is not parsed to see whether or not redirection is
        handled by the command, this option must be used with every shell
        command which includes any type of I/O redirection.

    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

        You cannot retry a flow command.

        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

        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.

        Each command can have a label attached to it which will allow you to
        refer to that command by the label. This is useful in analyzing the

        The label should not consist only of digits (i.e. be an integer).

    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

    In order to add the desired functionality, the commands are wrapped with
    some enclosing shell structure using very basic shell command which add
    the requested features. Simple command are very easy to wrap in a basic
    enclosing shell structure. For example, it is easy to turn:

       mycommand arg1 arg2


       if [ SOME_CONDITION ]; then
          mycommand arg1 arg2

    However, when commands are added which affect the flow of the script,
    they must be handled differently than simple commands in order to deal
    with them properly. Wrapping them in other shell structure would produce
    invalid shell scripts. As a result, each type of flow must be considered

    Currently, the supported flow commands are:

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

           if ... ; then
           elif ... ; then

        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 ...

        is not supported.

        The commands must be of the form:

           while ... ; do
           until ... ; do
           for ... ; do

    If flow commands are entered, but not correctly closed and/or nested, an
    error will be returned.

    Also, flow commands must generally be simple tests. If complex shell
    commands are entered which produce output, this output will NOT be
    handled correctly, and may actually break things when running in script

    At some point, the "select" and "case" structures may be supported, but
    this in not yet available.

    Also, shell functions are not currently supported.

    When commands are run in the 'script' mode, the output is stored in the

    To access the output, use one of the following calls to the output


    Each call to the method will return one part of the output. To following
    options may be used to determine what is returned.

        If the output was generated using the ssh method, this option is
        required (since it is possible to run the commands both locally and
        remotely, and the output is stored separately).

        HOST can be any of:

           all      The output for all hosts will be returned.  The return value
                    will be a hash of the form:

                       %ret = ( HOST1 => OUTPUT1, HOST2 => OUTPUT2, ...)

                    The output for all of the hosts listed will be returned in a

           HOST     If only a single host is specified, the output for only
                    that host will be returned.  It will not be returned as a hash.

        This tells what output will be returned. TYPE can be any of:

           stdout   STDOUT will be returned for the command(s) selected.
                    This is the default.

           stderr   STDERR will be returned for the command(s) selected.

           command  The command itself will be returned.

           num      The command number will be returned.

           label    The command label (if any) will be returned.

           exit     The exit code will be returned for the command(s) selected.
                    The exit code is the one returned by the final alternative
                    on the final try.

        This tells which commands will be included in the output. COMMAND
        can be any of:

           curr     An internal flag is kept which starts at the 1st command
                    which produced any type of output. The value is returned
                    for this command.  This is the default.

           next     The internal flag is incremented, and that becomes the new
                    current command.

           all      The value for all commands will be returned in the order they

           CMD_NUM  The commands are numbered starting at 1.  This will return the
                    output for only the command given.  Note however that a command
                    may occur multiple times (due to retries, being in a loop, etc.)
                    so the output will be a list of values, one per occurrence.

           LABEL    This will return the output for all commands assigned the
                    given label (using the per-command B<label> option).  Multiple
                    commands may be assigned the same label, so the output from
                    all occurrences of all commands with this label will be returned
                    as a list.

           fail     This will return the output for the command that failed (if any).

    Minimal support for complex scripts
        These methods work best for simple lists of commands. Using simple
        command flow (<if...then...else>, etc.) is allowed, but must be used
        carefully. The use of functions is NOT supported and will not work.

    Maximum of 200 commands fully supported
        In order to determine which command fails, a unique error code is
        assigned to each command. Since exit codes must be between 0-255,
        and some are reserved, there is a limit of 200 commands that can be
        entered if accurate error tracking is needed.

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

    Sullivan Beck (