PSUB(1) fish-shell PSUB(1)

# [NAME](#NAME)

psub - perform process substitution

# [SYNOPSIS](#SYNOPSIS)

```
COMMAND1 ( COMMAND2 | psub [-F | --fifo] [-f | --file] [(-s | --suffix) SUFFIX] )
```

# [DESCRIPTION](#DESCRIPTION)

Some shells (e.g., ksh, bash) feature a syntax that is a mix between command substitution and piping, called process substitution. It is used to send the output of a command into the calling command, much like command substitution, but with the difference that the output is not sent through commandline arguments but through a named pipe, with the filename of the named pipe sent as an argument to the calling program. **psub** combined with a regular command substitution provides the same functionality.

The following options are available:

[**-f** or **--file**](#f)

Use a regular file instead of a named pipe to communicate with the calling process. This will cause **psub** to be significantly slower when large amounts of data are involved, but has the advantage that the reading process can seek in the stream. This is the default.

[**-F** or **--fifo**](#F)

Use a named pipe rather than a file. You should only use this if the command produces no more than 8 KiB of output. The limit on the amount of data a FIFO can buffer varies with the OS but is typically 8 KiB, 16 KiB or 64 KiB. If you use this option and the command on the left of the psub pipeline produces more output a deadlock is likely to occur.

[**-s** or **--suffix** *SUFFIX*](#s)

Append SUFFIX to the filename.

[**-h** or **--help**](#h)

Displays help about using this command.

# [EXAMPLE](#EXAMPLE)

```
diff (sort a.txt | psub) (sort b.txt | psub)
# shows the difference between the sorted versions of files ``a.txt`` and ``b.txt``.
source-highlight -f esc (cpp main.c | psub -f -s .c)
# highlights ``main.c`` after preprocessing as a C source.
```

# [Copyright](#Copyright)

fish-shell developers

October 12, 2026 4.9

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Document: psub(1) from fish 4.9.3-r2 (Chainguard OS)
Source: https://github.com/fish-shell/fish-shell