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I have just seen this written down;

$ some-command >| /tmp/output.txt

Vertical pipes are used in standard redirects "piping" the output of one command to another, is >| in fact completely useless as it would be the same as just > in this scenario?

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up vote 181 down vote accepted

It's not useless - it's a specialised form of the plain > redirect operator (and, perhaps confusingly, nothing to do with pipes). bash and most other modern shells have an option noclobber, which prevents redirection from overwriting or destroying a file that already exists. For example, if noclobber is true, and the file /tmp/output.txt already exists, then this should fail:

$ some-command > /tmp/output.txt

However, you can explicitly override the setting of noclobber with the >| redirection operator - the redirection will work, even if noclobber is set.

You can find out if noclobber is set in your current environment with set -o.

For the historical note, both the "noclobber" option and its bypass features come from csh (late 70s). ksh copied it (early 80s) but used >| instead of >!. POSIX specified the ksh syntax (so all POSIX shells including bash, newer ash derivatives used as sh on some systems support it). Zsh supports both syntaxes. I don't think it was added to any Bourne shell variant but I might be wrong.

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Great answer, prompt and clear. Thanks very much! :D – jwbensley Aug 10 '12 at 16:13
I have also seen the symbol >! with the same meaning (using zsh) – PPC Aug 10 '12 at 17:01
It's equivalent to >! in csh and tcsh. – Keith Thompson Aug 10 '12 at 17:13
Wow. Learned something new today after more than 15 years of using linux... – Axel Aug 11 '12 at 10:11
@sch - re your historical note (thanks for the extra info, btw) - FreeBSD's sh has noclobber, since about May 2002. – D_Bye Sep 24 '12 at 10:42

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