Add xargs Default Command Is echo as a Unix TIL

unix/xargs-default-command-is-echo.md

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+# xargs Default Command Is echo
+
+With the `fd` command, all the found files are output line by line like so.
+
+```bash
+❯ fd '.*.md' --max-depth=1
+CONTRIBUTING.md
+README.md
+TODO.md
+built-in-commands.md
+newsletter.md
+tailwind.md
+```
+
+I can normalize the whitespace, squishing everything to be separated by single
+spaces by piping it to `xargs`. This is equivalent to call `xargs` with `echo`.
+That is because `echo` is the default command that `xargs` uses when another
+command isn't given.
+
+```bash
+❯ fd '.*.md' --max-depth=1 | xargs
+CONTRIBUTING.md README.md TODO.md built-in-commands.md newsletter.md tailwind.md
+
+❯ fd '.*.md' --max-depth=1 | xargs echo
+CONTRIBUTING.md README.md TODO.md built-in-commands.md newsletter.md tailwind.md
+```
+
+We can see further evidence of that by using the `-n` flag with `2` to have it
+process results two at a time. In either case, each pair of file names is
+echo'd before the next two are processed.
+
+```bash
+❯ fd '.*.md' --max-depth=1 | xargs -n2
+CONTRIBUTING.md README.md
+TODO.md built-in-commands.md
+newsletter.md tailwind.md
+
+❯ fd '.*.md' --max-depth=1 | xargs -n2 echo
+CONTRIBUTING.md README.md
+TODO.md built-in-commands.md
+newsletter.md tailwind.md
+```
+
+See `man xargs` for more details, as well as this [excellent
+article](https://abhinavg.net/2022/06/04/xargs-spaces/).