Keep a file open forever within a bash script

Question

I need a way to make a process keep a certain file open forever. Here's an example of what I have so far:

sleep 1000 > myfile &

It works for a thousand seconds, but really don't want to make some complicated sleep/loop statement. This post suggested that cat is the same thing as sleep for infinite. So I tried this:

cat > myfile &

It almost looks like a mistake doesn't it? It seemed to work from the command line, but in a script the file connection did not stay open. Any other ideas?

Answer 1

Rather than using a background process, you can also just use bash to open one of its file descriptors:

exec 5>myfile 

(The special use of exec here allows changing the current file descriptor redirections - see man bash for details). This will open file descriptor 5 to "myfile" (use >> if you don't want to empty the file).

You can later close the file again with:

exec 5>&-

(One possible downside of this is that the FD gets inherited by every program that the shell runs in the meantime. Mostly this is harmless - e.g. your greps and seds will generally ignore the extra FD - but it could be annoying in some cases, especially if you spawn any processes that stay around (because they will then keep the FD open).

Note: If you are using a newer version of bash (>4.1) you can use a slightly different syntax:

exec {fd}>myfile

This allocates a new file descriptor, and puts it in the variable fd. This can help ensure that scripts don't accidentally overwrite each other's file descriptors. To close the file later, use

exec {fd}>&-

Answer 2

On the cat > myfile & issue running in terminal vs not running as part of a script: In a non-interactive shell the stdin of a backgrounded command & gets implicitly redirected from /dev/null.

So, cat > myfile & in a script actually gets translated into cat </dev/null > myfile, which terminates cat immediately.

See the POSIX standard on the Shell Command Language & Asynchronous Lists:

The standard input for an asynchronous list, before any explicit redirections are 
performed, shall be considered to be assigned to a file that has the same 
properties as /dev/null. If it is an interactive shell, this need not happen. 
In all cases, explicit redirection of standard input shall override this activity.

---

# some tests
sh -c 'sleep 10 & lsof -p ${!}'
sh -c 'sleep 10 0<&0 & lsof -p ${!}'
sh -ic 'sleep 10 & lsof -p ${!}'


# in a script
-  cat > myfile &
+  cat 0<&0 > myfile &