(Solved) Ways to run the join command on memory/variable contents instead of files? (programming.dev)

submitted 2 weeks ago* (last edited 2 weeks ago) by [email protected] to c/[email protected]

3 comments fedilink hide all child comments

Edit: I figured it out: The solution is over a process substitution operator:

join <(echo "$var1") <(echo "$var2")

See also the comment by @[email protected] below, this StackOverflow comment, and the GNU documentation..

It's comparatively straightforward to use the content of one variable in join by using - to tell it to use standard input for that file:

echo $variable | join - anotherfile

However, is there a way to serve both input 'files' from variables, avoiding temporary files on the disk?

It seems like the easiest way would be via creating and mounting a temporary partition via tmpfs,

mount -t tmpfs -o size=50m tmpfs /mountpoint,

and just create temporary files in there. And afterwards clean things up.

So far I've also attempted here-documents, but apparently this too can only provide standard input, so that the other input still has to be served from a file.

Maybe one can also try doing it via named pipes (mkfifo), but I fear this could introduce lots of potentialities for errors.

top 3 comments

sorted by: hot top new old

[-] [email protected] 2 points 2 weeks ago

You can use the <( ) replacement for this as it runs a command in a subshell and replaces it with the filename of a temporary fifo. So something like:

join <( echo "${var1}" ) <( echo "${var2}" )

[-] [email protected] 1 points 2 weeks ago* (last edited 2 weeks ago)

Perfect, thanks for the explanation. Indeed, I found the same solution via StackOverflow about simultaneously.

[-] [email protected] 2 points 2 weeks ago* (last edited 2 weeks ago)

The bash man page is full of useful bits like this, but is is densely packed. It's definitely worth reading though.

this post was submitted on 12 Jul 2025

2 points (100.0% liked)

Shell Scripting

1508 readers

1 users here now

From Ash, Bash and Csh to Xonsh, Ysh and Zsh; all shell languages are welcome here!

Rules:

Follow Lemmy rules!
Posts must relate to shell scripting. (See bottom of sidebar for more information.)
Only make helpful replies to questions. This is not the place for low effort joke answers.
No discussion about piracy or hacking.
If you find a solution to your problem by other means, please take your time to write down the steps you used to solve your problem in the original post. You can potentially help others having the same problem!
These rules will change as the community grows.

Keep posts about shell scripting! Here are some guidelines to help:

Allowed: Release Announcement of a command-line program designed for scripted use (e.g. bash, bats, awk, jq, coreutils/moreutils)
Allowed: Tutorials on using shell languages or supplementary tools designed for scripted use
Allowed: Code review/help requests for shell languages or supplementary tools designed for scripted use
NOT Allowed: Announcement of a CLI or TUI program that is not designed for scripted use (Yes, your fancy Zsh plugin which pretty-prints the date and time using only builtins is very cool, but unless you actually want to discuss the code itself, please check out !commandline instead!)
NOT Allowed: Domain-specific tutorials that do not involve shell scripting as a core component (e.g. docker-compose, ansible, nix). If you really love one of these, I'm sure there's a community out there ready to talk about them!
NOT Allowed: Code review requests for non-shell programming languages and configuration languages (e.g. Python, Yaml)

In general, if your submission text is primarily shell code, then it is welcome here!

founded 2 years ago

MODERATORS

[email protected]