Distributed Systems Administration Utilities User's Guide, Linux, March 2009

4 Command Fanout
Command fanout utilities allow the system administrator to replicate shell commands across
multiple systems. Traditionally, administrators have created wrappers around tools such as
remote shell (see remsh(1)) and secure shell (see ssh(1)) to provide command fanout functions.
4.1 Parallel Distributed Shell
The Distributed Systems Administration Utilities (DSAU) include the open source tool Parallel
Distributed Shell (pdsh). pdsh formalizes the use of remsh and ssh for distributing commands
to groups of systems. Unlike remsh/ssh wrappers, pdsh offers the following benefits:
• High performance
Commands are issued in parallel to groups of target system. pdsh supports a sliding window
or fanout setting to control the number of concurrent commands.
• Command timeout settings
pdsh supports a command execution timeout which controls how long a remote command
can execute before being disconnected (to prevent problem commands from hanging). It
also supports a connect timeout which prevents blocking when remote systems are
unreachable.
• Output processing and return status
pdsh correctly handles stdout and stderr processing and supports returning a “worst
of”return status so the caller can detect errors from remote systems.
• Flexible target system specifications
pdsh supports several mechanisms for specifying the target hosts on which to operate. They
can be specified on the command line, on stdin, in a well known file (/etc/machines)
or in a file pointed to by the WCOLL environment variable. Specific systems can be excluded
from the command line as well.
• Hostlist expressions
For groups of systems using a prefixNNN naming convention (for example, h1, h2, ..., hN),
pdsh allows target nodes specification using hostlist expressions such as “h[1-10]” which
would fan out a command to hosts named h1 through h10.
• Intelligent output filtering
pdsh prefaces each line of output with the hostname of originating system. dshbak (see
dshbak(8)) is a filter that can format the standard pdsh output in several different ways. The
dshbak -c flag looks for output from different hosts that is identical and consolidates the
output instead of duplicating it. The header will indicate the hosts to which the consolidated
output applies.
• Choice of command transports
pdsh can use either remote shell rcmd (see rcmd(3)) or ssh as a command transport. Note
that the ssh transport offers greatly improved security. See “Security Configuration”
(page 85) for details.
4.1 Parallel Distributed Shell 83