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

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))"
  • I do not think I can achieve this with >> (output-redirection). According to this >> operation opens files with O_CREAT. – Hakan Baba Nov 19 '17 at 6:25

I think this should accomplish what you want:

sed -i '' -e '$a\
' 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.

  • I have not used sed much except for simple replace s/../../g. So sorry for uneducated questions. For gnu-sed I needed to remove the '' but not the -i. But in that case the gnu-sed appends one more extra line after Something Do you need the final new line in the sed script? I think I can work around the 0 bytes problem. I can make the file contain a new line at the start instead of being empty. Why is this command a NOOP for empty files? Is it because sed is line based and skips processing if it cannot find any lines ? – Hakan Baba Nov 19 '17 at 6:17
  • My issue with this solution is portability. I need the solution to work in BSD and in Linux. I will update the question accordingly. It should not be too difficult to modify the parameters and the sed script to generate the same output for bsd and gnu seds. – Hakan Baba Nov 19 '17 at 6:19

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