From 88fc5bc98370066490a9670c6b28fba91efd7d4d Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Tue, 20 Aug 2024 01:03:48 +0200
Subject: zsh,bash: trap handling

---
 src/shells/bash.md | 34 ++++++++++++++++++++++++++++++++++
 src/shells/zsh.md  | 35 ++++++++++++++++++++++++++++++++++-
 2 files changed, 68 insertions(+), 1 deletion(-)

diff --git a/src/shells/bash.md b/src/shells/bash.md
index f27b3a2..fb511b8 100644
--- a/src/shells/bash.md
+++ b/src/shells/bash.md
@@ -133,6 +133,40 @@ v=abc ; ( v=foo; echo $v; ) ; echo $v
 # abc
 ```
 
+## Trap Handling
+
+```bash
+trap "<CMDs>" <SIG>/EXIT
+
+# Show current trap handler.
+trap -p
+# List signal names.
+trap -l
+```
+
+#### Example: Run handler only on error exit
+```bash
+trap 'test $? -ne 0 && echo "run exit trap"' EXIT
+
+# -> no print
+exit 0
+# -> print
+exit 1
+```
+
+#### Example: Mutex in shell script
+For example if a script is triggered in unsynchronized, we may want to ensure
+that a single script instance runs.
+```bash
+# open file=LOCK with fd=100
+exec 100>LOCK
+# take exclusive lock, wait maximal for 3600s
+flock -w 3600 -x 100 || { echo "flock timeout"; exit 1; }
+
+# eg automatically release lock when script exits
+trap "flock -u 100" EXIT
+```
+
 ## Argument parsing with `getopts`
 
 The `getopts` builtin uses following global variables:
diff --git a/src/shells/zsh.md b/src/shells/zsh.md
index f702655..6be1967 100644
--- a/src/shells/zsh.md
+++ b/src/shells/zsh.md
@@ -238,6 +238,40 @@ if [[ $INPUT =~ $REGEX ]]; then
 fi
 ```
 
+## Trap Handling
+
+```zsh
+trap "<CMDs>" <SIG>/EXIT
+
+# Show current trap handler.
+trap -p
+# List signal names.
+trap -l
+```
+
+#### Example: Run handler only on error exit
+```zsh
+trap 'test $? -ne 0 && echo "run exit trap"' EXIT
+
+# -> no print
+exit 0
+# -> print
+exit 1
+```
+
+#### Example: Mutex in shell script
+For example if a script is triggered in unsynchronized, we may want to ensure
+that a single script instance runs.
+```zsh
+# open file=LOCK with fd=100
+exec 100>LOCK
+# take exclusive lock, wait maximal for 3600s
+flock -w 3600 -x 100 || { echo "flock timeout"; exit 1; }
+
+# eg automatically release lock when script exits
+trap "flock -u 100" EXIT
+```
+
 ## Completion
 
 ### Installation
@@ -366,6 +400,5 @@ Explanation:
 > - [zsh completion functions][zsh-comp-fn]
 > - [zsh completion utility functions][zsh-comp-utility-fn]
 
-
 [zsh-comp-fn]: http://zsh.sourceforge.net/Doc/Release/Completion-System.html#Completion-Functions
 [zsh-comp-utility-fn]: https://github.com/zsh-users/zsh-completions/blob/master/zsh-completions-howto.org#utility-functions
-- 
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