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How can I check if /proc/ is mounted?
Using /etc/mtab is discouraged as it might be inconsistent.
Using /proc/mounts is also not an option as might not exist if /proc/ is not mounted (although checking for its existence may be a way to do this check.
What is the best way to do this check?
You can run the mount without any arguments to get a list of current mounts. The /etc/mtab file should have similar data, but like you said it is possible for this to be inconsistent with what is actually mounted in the event that the /etc file system is messed up, not writable, or another program has messed with it. You can get specific information about the proc mounts by asking mount to list all mounts of type proc like this:
mount -l -t proc
Edit: It looks like you can use stat to compare the device of the /proc folder to the device of / to tell at least if SOMETHING is mounted there other than the root file system:
[[ $(stat -c %d%D /proc) != $(stat -c %d%D /) ]] && echo "Something is mounted at /proc"
mount | grep proc for just that reason.
[[ -n $(mount -l -t proc) ]]
Test for the existence of /proc/mounts. Running mount is no good because if /proc isn't mounted, it will return potentially obsolete data from /etc.
In theory there might be something else at /proc. But this is extremely unlikely in practice: if /proc/mounts exists and /proc isn't the proc filesystem, you can't trust anything about your environment anyway. If you're really worried, you can check that the filesystem type is proc: df -PT /proc | awk 'NR==2 && $2=="proc" {print 1}' (requires the Linux utilities df, there's no corresponding option in Busybox). Conversely, in theory, there could be a proc filesystem mounted in a different place; there's no easy way to find this with shell commands (df reads /proc/mounts to enumerate filesystems). In practice, just check for /proc/mounts.
If you want a (more or less) guarantee that the filesystem on /proc is in fact proc instead of some other filesystem or a directory structure made to look like it, you can call statfs() and check the type, 0x9fa0 is the value for proc.
There is tool called mountpoint avaiable on many linux installations.
The exitcode is 0 if the first argument is a mountpoint.
