Bash 4.0 introduced //coprocesses//, a feature certainly familiar to ksh users. The ''coproc'' keyword starts a command as a background job, setting up pipes connected to both its stdin and stdout so that you can interact with it bidirectionally. Optionally, the co-process can have a name ''NAME''. If ''NAME'' is given, the command that follows **must be a compound command**. If no ''NAME'' is given, then the command can be either simple or compound.
The process ID of the shell spawned to execute the coprocess is available through the value of the variable named by ''NAME'' followed by a ''_PID'' suffix. For example, the variable name used to store the PID of a coproc started with no ''NAME'' given would be ''COPROC_PID'' (because ''COPROC'' is the default ''NAME''). [[commands:builtin:wait]] may be used to wait for the coprocess to terminate. Additionally, coprocesses may be manipulated through their ''jobspec''.
==== Return status ====
The return status of a coprocess is the exit status of its command.
==== Redirections ====
The optional redirections are applied after the pipes have been set up. Some examples:
Here we need to save the previous file descriptor of stdout, because by the time we redirect the fds of the coprocess, stdout has already been redirected to the pipe.
==== Pitfalls ====
=== Avoid the final pipeline subshell ===
The traditional Ksh workaround to avoid the subshell when doing ''command | while read'' is to use a coprocess. Unfortunately, Bash's behavior differs.
By the time we start reading from the output of the coprocess, the file descriptor has been closed.
See [[http://mywiki.wooledge.org/BashFAQ/024 | this FAQ entry on Greg's wiki]] for other pipeline subshell workarounds.
=== Buffering ===
In the first example, we GNU awk's ''fflush()'' command. As always, when you use pipes the I/O operations are buffered. Let's see what happens with ''sed'':
Unlike ksh, Bash doesn't have true anonymous coprocesses. Instead, Bash assigns FDs to a default array named ''COPROC'' if no ''NAME'' is supplied. Here's an example:
This command starts in the background, and ''coproc'' returns immediately. Two new file descriptors are now available via the ''COPROC'' array. We can send data to our command:
Using a named coprocess is simple. We just need a compound command (like when defining a function), and the resulting FDs will be assigned to the indexed array ''NAME'' we supply instead.
* The ''coproc'' keyword is not specified by POSIX(R)
* The ''coproc'' keyword appeared in Bash version 4.0-alpha
* The ''-p'' option to Bash's ''print'' loadable is a NOOP and not connected to Bash coprocesses in any way. It is only recognized as an option for ksh compatibility, and has no effect.
* The ''-p'' option to Bash's ''[[commands:builtin:read | read]]'' builtin conflicts with that of all kshes and zsh. The equivalent in those shells is to add a ''\?prompt'' suffix to the first variable name argument to ''read''. i.e., if the first variable name given contains a ''?'' character, the remainder of the argument is used as the prompt string. Since this feature is pointless and redundant, I suggest not using it in either shell. Simply precede the ''read'' command with a ''printf %s prompt >&2''.
ksh93, mksh, zsh, and Bash all support something called "coprocesses" which all do approximately the same thing. ksh93 and mksh have virtually identical syntax and semantics for coprocs. A //list// operator: ''|&'' is added to the language which runs the preceding //pipeline// as a coprocess (This is another reason not to use the special ''|&'' pipe operator in Bash -- its syntax is conflicting). The ''-p'' option to the ''read'' and ''print'' builtins can then be used to read and write to the pipe of the coprocess (whose FD isn't yet known). Special redirects are added to move the last spawned coprocess to a different FD: ''<&p'' and ''>&p'', at which point it can be accessed at the new FD using ordinary redirection, and another coprocess may then be started, again using ''|&''.
zsh coprocesses are very similar to ksh except in the way they are started. zsh adds the shell reserved word ''coproc'' to the pipeline syntax (similar to the way Bash's ''time'' keyword works), so that the pipeline that follows is started as a coproc. The coproc's input and output FDs can then be accessed and moved using the same ''read''/''print'' ''-p'' and redirects used by the ksh shells.
It is unfortunate that Bash chose to go against existing practice in their coproc implementation, especially considering it was the last of the major shells to incorporate this feature. However, Bash's method accomplishes the same without requiring nearly as much additional syntax. The ''coproc'' keyword is easy enough to wrap in a function such that it takes Bash code as an ordinary argument and/or stdin like ''eval''. Coprocess functionality in other shells can be similarly wrapped to create a ''COPROC'' array automatically.
==== Only one coprocess at a time ====
The title says it all, complain to the bug-bash mailing list if you want more. See http://lists.gnu.org/archive/html/bug-bash/2011-04/msg00056.html for more details
The ability to use multiple coprocesses in Bash is considered "experimental". Bash will throw an error if you attempt to start more than one. This may be overridden at compile-time with the ''MULTIPLE_COPROCS'' option. However, at this time there are still issues -- see the above mailing list discussion.
