`entr`¶

entr is a tool available on Debian-based systems. It's used for running arbitraty commands when a file is changed.

This tool is not available in the default RedHat repositories. To get similar functionality on RedHat-based systems, look into inotify-tools, specifically inotifywait.

Note: If you're looking to use entr on a logfile to monitor for new logs or changes, you might want tail -F instead.

Using `entr` with a single file¶

entr expects the path of a file (or list of files) as standard input. This can be done with redirection < or with a pipe |.

The realpath command can be used to get the absolute path of a file if needed.

The entr program can rerun a program each time the given file is changed (written).

entr bash -c "clear; ./my_script" <<< my_script

This tells entr to run bash -c "clear; ./my_script" each time my_script is changed.
- Clears the screen, and then runs the file ./my_script.
<<< is a herestring operator (bash-only).
- It allows a string to be used as the standard input to a command.
- It means "take this string and send it to stdin as if it were a file."
- Not POSIX-compliant.
Alternatively, for a POSIX-compliant solution, pipe the output of an ls or find command to entr.
```
find . -type f -name 'my_script' | entr bash -c 'clear; ./my_script'
```
Another alternative would be using process substitution, using the <() syntax (bash-only).
```
entr bash -c "clear; ./my_script" < <(realpath ./my_script)
```

To automatically clear the screen, use the -c flag for entr (entr -c).
Then you don't have to call clear; in the command passed to bash.

entr -c bash -c "/tmp/t.sh;" < <(realpath /tmp/t.sh)  
# or  
realpath /tmp/t.sh | entr -c bash -c "/tmp/t.sh;"

Using `entr` with multiple files¶

You can set entr to run a command when any file in a list of files changes.
Pass in a list of files as input:

entr -c bash -c "./my_script" < <(find . -name '*.sh')

This will run ./my_script each time any file with a .sh extension
in the current directory or any subdirectoriesis changed.

Example with Golang Project Testing¶

entr bash -c "clear; go test -v ./..." < <(find . -name '*.go')  

# Or, with globstar enabled:  
entr bash -c "clear; go test -v ./..." < <(ls **/*.go)

The syntax ./... is Go's way of saying "all packages in the current directory
and subdirectories."

Other ways to automate¶

Using `watch`¶

Another option is watch.
This will (by default) run the given command every 2 seconds.
It will clear the screen for each iteration, so there's no need to put a clear; in.

watch "./my_script"

man watch

Infinite Loop (Generally a Bad Idea)¶

A noob option is to use a loop (poor man's watch).

while true; do ./my_script; sleep 1; done

This will run the script every second.
If you do this, remember to use sleep!

entr¶

Using entr with a single file¶

Using entr with multiple files¶