Process substitution

In computing, process substitution is a form of inter-process communication that allows the input or output of a command to appear as a file. The command is substituted in-line, where a file name would normally occur, by the command shell. This allows programs that normally only accept files to directly read from or write to another program.

Process substitution was available as a compile-time option for ksh88, the 1988 version of the Korn shell from Bell Labs. The Bash shell provided process substitution no later than version 1.14, released in 1994.

The following examples use Korn shell syntax.

The Unix diff command normally accepts the names of two files to compare, or one file name and standard input. Process substitution allows you to compare the output of two programs directly:

The <(command) expression tells the command interpreter to run command and make its output appear as a file. The command can be any arbitrarily complex shell command.

Without process substitution, the alternatives are:

1. Save the output of the command(s) to a temporary file, then read the temporary file(s).

2. Create a named pipe (also known as a FIFO), start one command writing to the named pipe in the background, then run the other command with the named pipe as input.

Both alternatives are more cumbersome.

Process substitution can also be used to capture output that would normally go to a file, and redirect it to the input of a process. The Bash syntax for writing to a process is >(command). Here is an example using the tee, wc and gzip commands that counts the lines in a file with wc -l and compresses it with gzip in one pass:

The main advantages of process substitution over its alternatives are:

Under the hood, process substitution has two implementations. On systems which support /dev/fd (most Unix-like systems) it works by calling the pipe() system call, which returns a file descriptor $fd for a new anonymous pipe, then creating the string /dev/fd/$fd, and substitutes that on the command line. On systems without /dev/fd support, it calls mkfifo with a new temporary filename to create a named pipe, and substitutes this filename on the command line. To illustrate the steps involved, consider the following simple command substitution on a system with /dev/fd support:

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