Meaning of “! -S” in shell script

The command you are looking at is actually this:

[ ! -S "qdata/c$i/tm.ipc" ]

Although it looks like punctuation, [ is actually the name of a command, also called test; so the command can also be written like this:

test ! -S "qdata/c$i/tm.ipc"

Which in context would look like this:

if test ! -S "qdata/c$i/tm.ipc"; then
        DOWN=true
fi

As the name suggests, its job is to test some attribute of a string, number, or file, and return 0 (which represents true in shell scripts) if the test passes, and 1 (which represents false) if it doesn’t.

Armed with this knowledge, you can run man test, and find the following explanations of the ! and -S arguments:

   ! EXPRESSION
         EXPRESSION is false

and

   -S FILE
         FILE exists and is a socket

So test ! -S filename or [ ! -S filename ] can be read as “not is-socket filename”.

So the command is checking whether a “socket” (a special kind of file) exists with each name in the loop. The script uses this command as the argument to an if statement (which can take any command, not just [) and sets DOWN to true if any of them does not exist.

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