atomic create file if not exists from bash script

Question

In system call open(), if I open with O_CREAT | O_EXCL, the system call ensures that the file will only be created if it does not exist. The atomicity is guaranteed by the system call. Is there a similar way to create a file in an atomic fashion from a bash script?

UPDATE: I found two different atomic ways

1. Use set -o noclobber. Then you can use > operator atomically.
2. Just use mkdir. Mkdir is atomic

Answer 1

A 100% pure bash solution:

set -o noclobber
{ > file ; } &> /dev/null

This command creates a file named file if there's no existent file named file. If there's a file named file, then do nothing (but return a non-zero return code).

Pros wrt the touch command:

• Doesn't update timestamp if file already existed
• 100% bash builtin
• Return code as expected: fail if file already existed or if file couldn't be created; success if file didn't exist and was created.

Cons:

• need to set the noclobber option (but it's okay in a script, if you're careful with redirections, or unset it afterwards).

I guess this solution is really the bash counterpart of the open system call with O_CREAT | O_EXCL.