man pipexec (1): create a directed graph of processes and pipes

SYNOPSIS

pipexec [OPTION]... [PROCESS DESCRIPTION]... [PIPE DESCRIPTION]...

DESCRIPTION

pipexec creates an arbitrary network (directed graph) of processes and pipes in between - even cycles are possible. It overcomes the shortcomings of shells that are typically only able to create non cyclic trees.

pipexec also monitors all it's child processes and is able to restart the whole network of processes and pipes if one crashes. Therefore pipexec can be used in SYSV-init or systemd configuration to run a network of processes.

OPTIONS

-h: print help and version information
-l logfd: use the given file descriptor for logging. If a 's' is specified, syslog is used. Example: Specifying '2' means log to stderr.
-p pidfile: with pipexec it is possible to handle pipes within SYSV-init scripts. In some environments (e.g. RHEL6, Debian7) the start and stop routines need a pid file. If this option is given, pipexec writes its own pid into the file shortly after start of pipexec.
-k: if one sub-process (child) gets killed and this options is given, all other sub-processes are also killed. Afterwards all processes are restarted.
-s sleep_time: the time interval in seconds before a restart. This option makes only sense when also the '-k' option is specified.

BACKGROUND

Inside a shell it is possible to start processes and redirect the output to other processes.

Example:

    cat Chap1.txt Chap2.txt | grep bird | wc -l

Three processes are created: the standard output (file descriptor (fd) 1) of the 'cat' process is connected to the standard input (fd 0) of the 'grep' command, and the standard output of the 'grep' command is connected to the standard input (fd 0) of the 'wc' process.

Please note that the assignment between names and file descriptor number is pure historical and has no technical background.

Example:

    find / 1> >(grep .txt) 2> >(wc >/tmp/w.log)

In this more complex example, the fd 1 of the 'find' process is connected to fd 0 of 'grep' and fd 2 is connected to fd 0 of 'wc'.

The limitation using this way of specifying processes and pipes is, that it is not possible to have any cycles. It is impossible to e.g. pass a fd of 'wc' either to 'grep' or to 'find'.

pipexec overcomes these limitations. It makes it possible to link any two arbitrary file descriptors in a set of processes.

USAGE

When building up a network of processes and pipes, there is the need to specify each element separately.

The processes will be the nodes in the network (directed graph), the connections of the file descriptors between to processes are the edges. Each node (process) has a unique name assigned to it. This makes it possible to differentiate between using the same command more than once.

The format of specifying a process is

    [ NAME /path/to/command arg1 arg2 ... argN ]

The first parameter 'NAME' must be a unique name. The second parameter must be the full path of the command to execute. Please note that always the full path must be specified, there is no PATH environment variable handling (execv(2) is used internally to span new processes). The following parameters are the parameters passed to the command.

The whole definition must be enclosed in square brackets. The square brackets must be given separately - before and after them must be a space.

The format of specifying a pipe between processes is

    {NAME_1:FD1>NAME_2:FD2}

Example

    {LS:1>GREP:0}

The names are the names of the processes, the numbers are the number of the file descriptor that should be used to build the pipe in between. When using pipexec from a shell (like bash) there is the need to escape the brackets or use quotation marks.

EXAMPLES

The shell command

    cat Chap1.txt Chap2.txt | grep bird | wc -l

is equivalent to

    pipexec [ CAT /bin/cat Chap1.txt Chap2.txt ] \
      [ GREP /usr/bin/grep bird ] [ WC /usr/bin/wc -l ] \
      "{CAT:1>GREP:0}" "{GREP:1>WC:0}"

The pipexec equivalent is longer and more complex in this example. But pipexec can build cycles that are impossible within a shell:

    pipexec [ A /bin/cmd1 ] [ B /bin/cmd2 ] "{A:1>B:0}" "{B:1>A:0}"

For more examples see the ptee(1) and peet(1) man pages.

AUTHOR

Written by Andreas Florath ([email protected])