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 KornShell from Bell Labs. The rc shell provides the feature as "pipeline branching" in Version 10 Unix, released in 1990. The Bash shell provided process substitution no later than version 1.14, released in 1994. The following examples use KornShell syntax. The Unix diff command normally accepts the names of two files to compare, or one file name and standard input. Process substitution allows one to compare the output of two programs directly: $ diff &2) < bigfile | gzip > bigfile.gz The main advantages of process substitution over its alternatives are: Simplicity: The commands can be given in-line; there is no need to save temporary files or create named pipes first. Performance: Reading directly from another process is often faster than having to write a temporary file to disk, then read it back in. This also saves disk space. Parallelism: The substituted process can be running concurrently with the command reading its output or writing its input, taking advantage of multiprocessing to reduce the total time for the computation.