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Thunderbird doesn't open files that don't have .eml extension as email files, but instead starts to compose a new message and adds them as attachments. To use thunderbird in scripts I'm looking for a way that would allow you to "temporarily rename" a file as a file.eml, open it in thunderbird (possibly edit, but at least read) and close it without saving anything to the disk - all in that process' memory.

While this could be achieved by either copying it or creating a symlink to that file in a temporary location there are many elegant ways to use process substitution or here strings that instead create "temporary files" in RAM that live only for duration of the process.

Is there anything that could create such "pseudo symlinks" in bash/zsh?

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    Related: Specifying the file extension produced by zsh process substitution. I think you can use like TMPPREFIX=/dev/shm/zsh, TMPSUFFIX=.eml and =(<file). It uses a regular file (a copy of your file), not a symlink, therefore not an answer. Commented Jun 6, 2023 at 21:49
  • Fantastic, thank you. I've had no idea that zsh has a file-based process substitution. I think this solves the problem. One new problem with this approach though is that the file gets deleted before thunderbird has time to read it (I suppose that it could be solved by launching it somehow in a subshell)? TMPSUFFIX=.eml; thunderbird =(<PATH/TO/THE/FILE) results in File not found in Tbird Commented Jun 6, 2023 at 22:16
  • UPDATE: adding & at the end solved it: TMPSUFFIX=.eml; thunderbird =(<PATH/TO/THE/FILE) & but now /tmp/zsh... files don't get deleted after processes exit Commented Jun 6, 2023 at 22:27
  • The file should remain until thunderbird exits. I guess if thunderbird spawns a child or delegates the job to another (already running) process and exits before the other process reads the file, then the other process may try to read after the file is deleted. The point is zsh deletes the file just after it notices the thunderbird exits. This is exactly "live only for duration of the process" you wanted. Disowning the command (with &!) results in zsh not noticing (and not caring) when the thunderbird exits, the file will remain. Totally not "only for duration of the process". Commented Jun 6, 2023 at 22:39
  • Thanks again, I didn't foresee this problem. This has been really helpful already and I don't know if there is any way for a shell to clean temporary files for such forking processes after itself. I suppose you may set up a cron job that will regularly run rm /dev/shm/zsh* on every leftover file older than some chosen age? Commented Jun 7, 2023 at 0:06

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If you're proposing a script that performs the sequence:

  1. Link filename to filename.eml
  2. Thunderbird opens filename.eml and does whatever it does with the data
  3. Undo the link from Step 1

Then why wouldn't a hard link have the desired effect? E.g., ln instead of ln -s:

file_name=/path/to/file ln "${file_name}" "${file_name}.eml" thunderbird -arg1 -arg2 -arg-etc rm "${file_name}.eml"

Hard links don't consume additional space on disk, they just provide an additional filename/filepath to the file's inode and data. No need to do the work of copying the file data into RAM unless the disk I/O is a problem (but it would have been a problem if the file had a .eml extension in the first place).

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  • I'm afraid this very thunderbird may exit before the file is read. If so, your rm may be premature. See the comments under the question; what happened to temporary files may happen to your temporary hardlinks. Commented Jun 6, 2023 at 23:05

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