Bash shell read error: 0: Resource temporarily unavailable

Question

When writing a bash script. Sometimes you are running a command which opens up another program such as npm, composer.. etc. But at the same time you need to use read in order to prompt the user.

Inevitable you hit this kind of error:

read: read error: 0: Resource temporarily unavailable

After doing some research there seems to be a solution by piping the STDIN of those programs which manipulate the STDIN of your bash script to /dev/null.

Something like:

npm install </dev/null

Other research has shown it has something to do with the fact that the STDIN is being set to some sort of blocking/noblocking status and it isn't being reset after the program finishes.

The question is there some sort of fool proof, elegant way of reading user prompted input without being affected by those programs that manipulate the STDIN and not having to hunt which programs need to have their STDIN redirected to /dev/null. You may even need to use the STDIN of those programs!

Answer 1

Usually it is important to know what input the invoked program expects and from where, so it is not a problem to redirect stdin from /dev/null for those that shouldn't be getting any.

Still, it is possible to do it for the shell itself and all invoked programs. Simply move stdin to another file descriptor and open /dev/null in its place. Like this:

exec 3<&0 0</dev/null

The above duplicates stdin file descriptor (0) under file descriptor 3 and then opens /dev/null to replace it.

After this any invoked command attempting to read stdin will be reading from /dev/null. Programs that should read original stdin should have redirection from file descriptor 3. Like this:

read -r var 0<&3

The < redirection operator assumes destination file descriptor 0, if it is omitted, so the above two commands could be written as such:

exec 3<&0 </dev/null
read -r var <&3

Answer 2

When this happens, run bash from within your bash shell, then exit it (thus returning to the original bash shell). I found a mention of this trick in https://github.com/fish-shell/fish-shell/issues/176 and it worked for me, seems like bash restores the STDIN state. Example:

bash> do something that exhibits the STDIN problem
bash> bash
bash> exit
bash> repeat something: STDIN problem fixed