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 are available in any bourne shell variation) 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 (typically with a command line option in the calling
script).
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. 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.
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 either a string containing a command, or a
listref where each element in the list is a string containing 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. Commands must not include
a trailing semi-colon as these will interfere with I/O redirection,
and 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. Refer to
the "FLOW COMMANDS" section below. Defining functions is NOT
supported.
run
$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()"
call.
The error code returned is described below in the "ERROR CODES"
section.
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.
output
$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
section.
flush
$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
given:
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
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. This is a shortcut
for:
$err = $obj->options('dire',$dire);
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. This is a shortcut
for:
$err = $obj->options('mode',$dire);
You can also check the value that is set using:
$mode = $obj->mode();
env
$obj->env($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();
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.
ERROR CODES
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
failed.
201 If more than 200 commands are entered,
and any of them beyond the 200th fail,
the error code will be 201.
252 An error in the script. Usually this
indicates that 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.
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/ssh methods. The following values are available.
run (default)
dry-run
script
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.
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 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.
output
The output option can be one of the following:
both (default)
merged
stdout
stderr
quiet
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).
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 (default)
script
simple
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.
echo
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:
echo
noecho
failed
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.
failure
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:
exit
display
continue
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
command.
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:
/tmp/.cmd.shell.$$
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.
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 and additional hosts will be run only after others have
completed.
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.
If it is set to a negative value -N, it will sleep for exactly N
seconds.
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 "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
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 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
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.
label
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
output.
The label should not consist only of digits (i.e. be an integer).
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.
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
into
if [ SOME_CONDITION ]; then
DO_SOMETHING
mycommand arg1 arg2
DO_SOMETHING
fi
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
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
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
mode.
At some point, the "select" and "case" structures may be supported, but
this in not yet available.
Also, shell functions are not currently supported.
OUTPUT OPTIONS
When commands are run in the 'script' mode, the output is stored in the
object.
To access the output, use one of the following calls to the output
method:
$obj->output(%options);
Each call to the method will return one part of the output. To following
options may be used to determine what is returned.
host=HOST
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, ...)
HOST1,HOST2,...
The output for all of the hosts listed will be returned in a
hash
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.
output=TYPE
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.
command=COMMAND
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
occurred.
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).
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. 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.
LICENSE
This script is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
AUTHOR
Sullivan Beck (
[email protected])
