Check if a Device or File System is Mounted

Occasionally I find myself needing to mount a remote file system on a local *BSD or Linux machine. On one such occasion recently I had mounted an NFS file system from a Network Attached Storage (NAS) server to a local machine running FreeBSD, for the purpose of backing up some of the files to yet another machine offsite using the rsync utility. I had created a little script to run rsync periodically through cron. This script worked well until I noticed a few days later that the backups on the remote machine no longer existed. After some investigation I quickly determined that NAS file system was no longer mounted on the FreeBSD machine (I can neither confirm or deny that I may have configured something incorrectly). The rsync script, upon noticing that files no longer existed in the source, deleted said files at the target. It was at that point that I decided to include a small test in the rsync script to check on the existence of the mounted file system; then, if it still existed, the script would proceed with the rsync command, else it would write an error message to a log. Here is the code snippet I used in the script, which you can easily adapt for other situations:

The df command simply displays statistics about the amount of free disk space on the specified file system. If a file system argument is not specified, statistics for all mounted file systems are displayed. The output of the df command is then piped through the venerable grep utility using its -q option, which instructs grep not to write anything to standard output, but rather exit immediately with a zero status if a match is found. Finally, the file system we’re interested in matching to, parsed from the output of the df command, is provided to grep. If the output of this df command is zero, the script runs a command (in this case my rsync command), else if it exits with anything other than zero, the script runs another command instead (in this case an error message).