How to make appends to files fail if the file does not exist already?

Question

For example

echo Something >> SomeFile

I want this append to work only if SomeFile exists already.

Right now I am using the following:

if [ -e SomeFile ]; then 
    echo Something >> SomeFile
fi; 

But there should be a race condition here. During the if condition evaluation SomeFile may exist. A context switch happens between the if condition and the append. Some other application executes that removes SomeFile. In that case the append would create SomeFile.

I need to solution to work for both bsd sed and gnu sed.

---

It is pretty simple to do this in python with os.open and O_APPEND

$ rm SomeFile
$ python -c "import os; print(os.open(\"SomeFile\",os.O_APPEND))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
OSError: [Errno 2] No such file or directory: 'SomeFile'
$ touch SomeFile
$ python -c "import os; print(os.open(\"SomeFile\",os.O_APPEND))"
3

Answer 1

I think this should accomplish what you want:

sed -i '' -e '$a\
Something\
' SomeFile

I did this with mac osx/bsd sed so you may be able to remove the -i '' part. This will fail if SomeFile does not exist however a caveat to this is it will also fail if the file exists and is 0 bytes, hopefully that isn't a deal breaker for you.