3 files changed, 27 insertions, 0 deletions
diff --git a/src/tools/bash.md b/src/tools/bash.md
index 9150963..a2b7944 100644
--- a/src/tools/bash.md
+++ b/src/tools/bash.md
@@ -104,6 +104,14 @@ command 2>&1 >file
 1. duplicate `fd 1` to `fd 2`, effectively redirecting `stderr` to `stdout`
 2. redirect `stdout` to `file`
 
+## Process substitution ([ref][psub])
+
+Process substitution allows to redirect the stdout of multiple processes at
+once.
+```bash
+vim -d <(grep foo bar) <(grep foo moose)
+```
+
 ## Argument parsing with `getopts`
 
 The `getopts` builtin uses following global variables:
@@ -227,3 +235,4 @@ complete -F _foo foo
 ```
 
 [dup2]: http://man7.org/linux/man-pages/man2/dup.2.html
+[psub]: https://tldp.org/LDP/abs/html/process-sub.html
diff --git a/src/tools/fish.md b/src/tools/fish.md
index 117ead2..fd1b2f7 100644
--- a/src/tools/fish.md
+++ b/src/tools/fish.md
@@ -87,6 +87,12 @@ echo "ls output: "(ls)
 echo foo >? log
 ```
 
+### Process substitution
+Redirect output of multiple processes. Same as `<(..)` in bash.
+```sh
+diff (sort a | psub) (sort b | psub)
+```
+
 ## Control Flow
 ### `if` / `else`
 ```sh
diff --git a/src/tools/zsh.md b/src/tools/zsh.md
index a1302a4..f702655 100644
--- a/src/tools/zsh.md
+++ b/src/tools/zsh.md
@@ -174,6 +174,18 @@ echo ${(kv)vec}       # a aa b bb
 for k v in ${(kv)vec)}; do ...; done
 ```
 
+## I/O redirections
+
+See [bash - I/O redirection](bash.md#io-redirection)
+
+## Process substitution
+
+Process substitution allows to redirect the stdout of multiple processes at
+once.
+```bash
+vim -d <(grep foo bar) <(grep foo moose)
+```
+
 ## Argument parsing with `zparseopts`
 
 ```zsh