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John Robertson
John Robertson

Posted on • Updated on

Event Driven Bash

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Bash is very useful when it comes to automating system administration tasks. Sometimes you need to take action based on external events, and there aren't a lot of examples of how this can be done. It's pretty straightforward:

#!/bin/bash -eu

# Launch inotifywait monitoring the syslog in a subprocess.
# Redirect stdout of subshell to pipe #3
exec 3< <(exec inotifywait -m /var/log/syslog)

# Read each line of output from inotifywait
while read -u 3 FILE OPS; do

   # stdin, stdout, stderr all available in loop

   echo "FILE= '$FILE', OPS= '$OPS'"

   # OPS are comma separated. Swap comma for space, deal with each individually.
   for op in ${OPS//,/ }; do

      # Branch on $op
      case $op in

            echo "$FILE was modified.";;

            echo "$FILE was accessed.";;

            echo "$FILE was closed without changes."
            break 2;; 

# Other actions go here

# Close pipe
exec 3<&-

# Only get here on loop exit, or if inotifywait quits.
exit 0

To exercise the script, try running it and then pulling up your syslog in a pager. When you exit the pager, your script should exit as well.
This and other examples are available on Github.
I'll be happy to try and answer any questions.

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