echo > file complains file does not exists

Question

I have a virtual machine running and its root directory mounted to in ./vm

I have this script running on the virtual machine:

#!/usr/bin/env bash
flag=/dir/flag
echo "Listening..."
while true; do
    while [[ ! -f "$flag" ]]; do
        sleep 2
    done;
    cat "$flag"
    rm -f "$flag"
done

In the host, I have this:

#!/usr/bin/env bash
arrayA=( a b c d )
arrayB=( w x y z )
while true; do
    for i in $(seq 0 $((${#arrayA[@]} - 1 ))); do
        rm -rf ./vm/dir/*
        sync
        echo "${arrayB[i]}" > ./vm/dir/flag
        read -p "press [Enter]"
    done
done

The problem I get, is that sometimes the script in the host yells at me:

./host.sh: line ..: vm/dir/flag: No such file or directory

I don't know how to fix this. I've inserted syncs here and there, but sometimes (not always) after pressing [Enter], I get the error message, the flag is not written, and the virtual machine does nothing.

Answer 1

You seem to have a race condition. The host does 1/ expand ./vm/dir/* then 2/ rm the found files. If the vm has deleted the file between 1/ and 2/, the host indicates an error.

It shouldn’t be frequent (I was surprised it could happens so frequently given the timing : sleep, read), but it is not impossible.

Note : you don’t need to rm the flag if you want to write another content. Just directly execute echo … > ./vm/dir/flag (You may have an issue : this involves two steps : reopening the file, writing to it. The best way to avoid race conditions seems to create a flag2 file and when it is ready, rename it flag. But here using cat on an empty file doesn’t seem to harm)

Edit : is your echo or your rm which indicates an error ? Echo should only indicates an error if the directory is not there.