Unix tip

[kpreid]

The following command is not idempotent:

cat * > cat.txt

Comments

[kpreid.livejournal.com]

I didn't notice that something was really wrong until the print-preview finally came up and told me there were 23k pages...

Also: the command would fail the first time if the shell expanded wildcards after performing redirections, instead of the other ordering.

[virkony.livejournal.com]

but "cat * > .cat.txt" is idempotent in most of shells

[(Anonymous)]

ln -s /bin/cat cat.txt