Last Updated: 2024-11-23

The following piece of code has an effect similar to rsync for copying folders with metadata like permissions. But what's exactly is going on with the combination of brackets and parentheses?

(cd src && tar -c .) | (cd dst && tar -xp)

Analysis of main stuff

• the kernel uses fork on each area in parentheses and each of these is known as a subshell. Each fork has its own working directory, own set options, etc. These processes don't interfere with one another.
• There are three processes in total (one parent, two children)
• Note that the pipe belongs to the parent process, not to either of the child processes
• Note too that the subshells produce STDOUT that the parent sees - e.g. if you run (echo "hi") > temp.txt && cat temp.txt you'll see "hi" written
• be careful of the difference between (echo "hi")and $(echo "hi"). In the latter case, it fails saying the command "hi" was not found - i.e. the parent shell tries to execute the returned value. This $() technique is known as "command substitution" and it also invokes a subshell.

You might want to supress an environmental variable temporarily with a subshell bash (unset http_proxy; wget ...)

Aside on the tar command

• -c bottle up argument directory (.) in tar
• -xp extracts contents preserving permissions

Resources

• https://mywiki.wooledge.org/SubShell
• https://www.destroyallsoftware.com/screencasts/catalog/tar-fork-and-the-tar-pipe
• https://unix.stackexchange.com/questions/147420/what-is-in-a-command