Any script to notify when there is no write activity in folder

Question

I am trying to come up with a nice and easy way of detecting when there has not been any write activity in a folder I'd like to watch.

Basically, what I'd like to have is something like this:

#!/bin/sh

# colored red text (error)
function errorMsg () {
  echo '\033[0;31m'"$1"'\033[0m'
}

# check for folder to monitor argument
if [ "$#" -ne 1 ]
then
  errorMsg "add experiment (folder) to monitor for activity!"
exit
fi

# time out time, 3 minutes
TIMEOUT_TIME=180
LAST_CHECKED=0

function refreshTimer () {
  # when called, check seconds since epoch
  CURRENT_TIME=date +%s

  if [ CURRENT_TIME - LAST_CHECKED > TIMEOUT_TIME ]
  then
    echo "file write activity halted!" | mail -s "trouble!" "[email protected]"
  fi

  LAST_CHECKED=date +%s
}

# set last checked to now.
LAST_CHECKED=date +%s
# start monitoring for file changes, and update timer when new files are being written.
fswatch -r ${1} | refreshTimer

but all sorts of bash magic is required I presume, since fswatch is a background task and piping its output creates a subshell. I would also be in need of some timer logic... I was thinking something like a setTimeout of which the time argument keeps being added to when there IS activity, but I don't know how to write it all in one script.

Bash, Python, Ruby, anything that can be installed using homebrew on OSX is fine but the simpler the better (so I understand what is happening).

Answer 1

Try the following - note that it requires bash:

#!/usr/bin/env bash

# colored red text (error)
function errorMsg () {
  printf '\033[0;31m%s\033[0m\n' "$*" >&2
}

# check for folder to monitor argument
if [[ $# -ne 1 ]]
then
  errorMsg "add experiment (folder) to monitor for activity!"
  exit 2
fi

# time-out: 3 minutes
TIMEOUT_TIME=180

# Read fswatch output as long as it is
# within the timeout value; every change reported
# resets the timer.
while IFS= read -t $TIMEOUT_TIME -d '' -r file; do 
  echo "changed: [$file]"
done < <(fswatch -r -0 "${1}")

# Getting here means that read timed out.
echo "file write activity halted!" | mail -s "trouble!" "[email protected]"

• fswatch indefinitely outputs lines to stdout, which must be read line by line to take timely action on new output.
• fswatch -0 terminates lines with NULs (zero bytes), which read then reads on be one by setting the line delimiter (separator) to an empty string (-d '').
• < <(fswatch -r -0 "${1}") provides input via stdin < to the while loop, where read consumes the stdin input one NUL-terminated line at a time.
  • <(fswatch -r -0 "${1}") is a process substitution that forms an "ad-hoc file" (technically, a FIFO or named file descriptor) from the output produced by fswatch -r -0 "${1}" (which watches folder ${1}'s subtree (-r) for file changes, and reports each terminated with NUL (-0)).
  • Since the fswatch command runs indefinitely, the "ad-hoc file" will continue to provide input, although typically only intermittently, depending on filesystem activity.
• Whenever the read command receives a new line within the timeout period (-t $TIMEOUT_TIME), it terminates successfully (exit code 0), causing the body of the loop to be executed and then read to be invoked again.
  • Thus, whenever a line has been received, the timeout period is effectively reset, because the new read invocation starts over with the timeout period.
  • By contrast, if the timeout period expires before another line is received, read terminates unsuccessfully - with a nonzero exit code indicating failure, which causes the while loop to terminate.
• Thus, the code after the loop is only reached when the read command times out.

---

As for your original code:

Note: Some of the problems discussed could have been detected with the help of shellecheck.net

• echo '\033[0;31m'"$1"'\033[0m'
  • printf is the better choice when it comes to interpreting escape sequences, since echo's behavior varies across shells and platforms; for instance, running your script with bash will not interpret them (unless you also add the -e option).
• function refreshTimer ()
  • The function syntax is nonstandard (not POSIX-compliant), so you shouldn't use it with a sh shebang line (that's what chepner meant in his comment). On OSX, you can get away with it, because bash acts as sh and most bashisms are still available when run as sh, but it wouldn't work on other systems. If you know you'll be running with bash anyway, it's better to use a bash shebang line.
• CURRENT_TIME=date +%s
  • You can't assign the output from commands to a variable by simply placing the command as is on the RHS of the assignment; instead, you need a command substitution; in the case at hand: CURRENT_TIME=$(date +%s) (the older syntax with backticks - CURRENT_TIME=`date +%s` - works too, but has disadvantages).

• [ CURRENT_TIME - LAST_CHECKED > TIMEOUT_TIME ]

  • > in [ ... ] and bash's [[ ... ]] conditionals is lexical comparison and the variable names must be $-prefixed; you'd have to use an arithmetic conditional with your syntax: (( CURRENT_TIME - LAST_CHECKED > TIMEOUT_TIME ))
  • As an aside, it's better better not to use all-uppercase variable names in shell programming.

• fswatch -r ${1} | refreshTimer

  • refreshTimer will not get called until the pipe fills up (the timing of which you won't be able to predict), because you make no attempt to read line by line.
  • Even if you fix that problem, the variables inside refreshTimer won't be preserved, because refreshTimer runs in a new subshell every time, due to use of a pipeline (|). In bash, this problem is frequently worked around by providing input via a process substitution (<(...)), which you can see in action in my code above.