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
attempt 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 supported flow commands are:
"if...elsif...else...fi"
In order to recognize them, the commands will be partially 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.
"while...done"
"until...done"
"for...done"
The commands must be of the form:
while ... ; do
until ... ; do
for ... ;do
done
At some point, the "select" and "case" structures may be supported, but
this in not yet available.
KNOWN PROBLEMS
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.
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)