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content/blog/command-when-file-in-directory-changes.md

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+---
+title: "Run a command when a file in a directory changes"
+date: 2024-01-05T16:00:53-05:00
+draft: false
+tags: []
+math: false
+medium_enabled: false
+---
+
+When editing code or adding content to a HTML file for a website, it can be very useful to see the changes right away after you save the file. We'll show a lightweight way of accomplishing this task with `entr` and a bit of bash scripting.
+
+First if you don't already have it, install `entr`.
+
+```bash
+sudo dnf install entr
+```
+
+This terminal application works by monitoring files passed within standard in. Therefore, to run a `$command`  if any file under some `$directory` changes run the following:
+
+```bash
+find $directory -type f | entr $command
+```
+
+An example can be running a build script if anything under the source folder changes:
+
+```bash
+find src -type f | entr ./build.sh
+```
+
+Now this doesn't capture files that get added to the directory. To do this, we need to get `entr` to monitor the parent folder for any changes. We can do this with the `-d` flag.
+
+From the man page:
+
+> **-d**    Track the directories of regular files provided as input and exit if 
+>        a new file is added. 
+
+Since `entr` exits when a file gets added with this flag, the [common solution](https://superuser.com/a/665208) on the internet is to wrap it in a `while` loop.
+
+```bash
+$ while sleep 0.1 ; do find $directory -type f | entr -d $command ; done
+```
+
+This solution works great as a one-liner, but it doesn't let me CTRL-C out when I'm finished. Therefore, I wrote a shell script that incorporates this solution while also adding a `trap`.
+
+```bash
+#!/bin/bash
+
+set -o nounset
+set -o pipefail
+
+show_usage() {
+    echo "Usage: entr-dir [dir] [command]"
+    exit 1
+}
+
+# Check argument count
+if [ "$#" -lt 2 ]; then
+    show_usage
+fi
+
+# Make sure that the command entr exists
+if ! command -v entr > /dev/null ; then
+    echo "entr not found. Exiting..."
+    exit 1
+fi
+
+DIR="$1"; shift
+
+if [[ ! -d "$DIR" ]]; then
+    echo "First argument must be a directory. Exiting..."
+    exit 1
+fi
+
+# Allow for CTRL-C to exit script
+trap "exit 0;" SIGINT
+
+while sleep 0.1; do
+    find "$DIR" -type f | entr -d "$@"
+done
+```
+
+I plopped this in `~/.local/bin/entr-dir`, gave it the `+x` permission, and now I can easily monitor and build projects when files get changed/added/deleted.
+
+```bash
+entr-dir src ./build.sh
+```
+