Bash one-liner to atomically create a file if it doesn't exist

Question

I want a Bash one-liner that atomically creates a file if it doesn't exist. That means essentially "if file doesn't exist, create it" but ensuring that no one else manages to create the file in the small space between doing the if and creating the file.

Sorry, I read 'atomically' as 'automatically.' I deleted my answer. — Aaron Copley, May 01 '16 at 13:19
Did you see this; [atomic create file if not exists from bash script](http://stackoverflow.com/q/13828544/1207760)? — Aaron Copley, May 01 '16 at 13:22

score 8 · Answer 1 · edited Jun 12 '17 at 17:51

8

Stealing an answer from various comments and links to [SO]. It seems there is a POSIX compliant method that doesn't involve mkdir as I mentioned in my original answer below

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

This redirect to file returns 0 or fails and returns non-zero if the file already exists.

Original answer

You'll have to use mkdir - that's atomic, either the directory gets created and you can continue or it doesn't created so you take appropriate action.

Of course, mkdir doesn't create a file but once you know you have exclusive access to the directory then you can make the file you want in it.

As to a one liner - I'll leave that up to you. Personally I'd write it over a few lines, as that'll be more maintainable.

edited Jun 12 '17 at 17:51

quadruplebucky

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answered May 01 '16 at 13:58

user9517

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But noclobber from http://stackoverflow.com/q/13828544/1207760 is more what OP wants. – Law29 May 01 '16 at 15:50
Noclobber is, in general, a good answer. But it doesn't work for ksh88. – jrw32982 May 03 '16 at 20:44
1

@JohnWiersba but the question is about bash. – womble May 25 '16 at 01:58
@womble yes but the answer mentions POSIX which someone might take to mean that the answer applies to other shells. Ksh88 is still the standard shell on some servers. – jrw32982 Jun 03 '16 at 15:25
Does this technique also work for bash in standard (non-posix) mode? – Binarus Nov 17 '20 at 09:39

score 1 · Answer 2 · answered May 01 '16 at 21:09

1

Try this one. The ln provides the test-and-set functionality.

touch lock.$$.tmp
if ln lock.$$.tmp lock.dat 2>/dev/null
then
    echo "File is mine"
else
    echo "Test and set failed"
fi
rm -f lock.$$.tmp

answered May 01 '16 at 21:09

roaima

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score 0 · Answer 3 · answered May 01 '16 at 14:00

0

Is it file or directory? If file, you can use simple command touch - if file exists, it just modify last access time. If file doesn't exists, is created.

answered May 01 '16 at 14:00

Ondra Sniper Flidr

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score 0 · Answer 4 · answered Jun 05 '16 at 19:57

If you don't care about the filename, you can delegate this task to a utility, for example mktemp

Create a temporary file or directory, safely, and print its name.
TEMPLATE must contain at least 3 consecutive `X's in last component.
If TEMPLATE is not spec‐ified, use tmp.XXXXXXXXXX, and --tmpdir is 
implied.  Files are created u+rw, and directories u+rwx, minus umask
restrictions.

Invocation:

filename=$(mktemp)

Bash one-liner to atomically create a file if it doesn't exist

4 Answers4