Detect staled NFS mount

Here's a simple script to check whether the given directory (NFS mount point) is stale.

There are three points that needs some explanation here.

First, since any command that access the NFS file system would block (unresponsive) iff the NFS is stale, I am using read -t N for the timeout.

Second, I used process substitution feature of bash, <(list) form. Basically, read -t 1 < <(...) will timeout after 1 second unless =⁠…= part finished within the timeout. bash will create a new subshell to execute /⁠list/ in <(list) form. The problem is, if any of list will access the stale NFS, the command will hang, which makes the subshell also will hang. Even if the calling shell script finished, the subshell would not terminated, leaving the process in interruptible sleep state.

To prevent this, I recorded the sub-shell PID using $BASHPID in <(list) form. After =⁠read= command, I deleted all possible children of the subshell and the subshell itself. Note that using $$ won't work in <(list) form. =⁠$$= represents the mother shell's PID, not the sub-shell's.