Feature: disallow | >
In bash, the empty command can be redirected to a file: % >out It will create or overwrite 'out' as the empty file. That may be useful. But somehow you can also say % whoami | >out This again writes empty content. I suggest this odd feature is not useful and indeed gets in the way, since when making pipelines you may mistakenly put an extra | at the last step before the output redirection. Bash should forbid the odd syntax with | immediately followed by >. If you have | it must be followed by a command. Note that in most cases this is already required: % | # syntax error % whoami | # syntax error -- Ed Avis __ This email has been scanned by the Symantec Email Security.cloud service. For more information please visit http://www.symanteccloud.com __
Re: Feature: disallow | >
On Tue, Mar 17, 2015 at 11:40 AM, Ed Avis wrote: > In bash, the empty command can be redirected to a file: > > % >out > > It will create or overwrite 'out' as the empty file. That may be useful. > But somehow you can also say > > % whoami | >out This isn't surprising or deserving of a special case treatment. It's a valid permutation that I would expect to work as required. This isn't even close to the most likely way one might screw up a redirect. Forgetting a quote or escape in a non-obvious place such as within an assignment or argument to a command is far more likely to bite you. Examples: var=foo>out let x>5\?y:z If you're worried about making a mistake, enable the noclobber option in your bashrc. -- Dan Douglas
RE: Feature: disallow | >
The reason I found 'whoami | >out' to be surprising is that none of the following work: % | >out % (whoami |) % whoami | | cat If | > is a valid construct, what are its semantics? -- Ed Avis __ This email has been scanned by the Symantec Email Security.cloud service. For more information please visit http://www.symanteccloud.com __
Re: Feature: disallow | >
Ed Avis writes: > If | > is a valid construct, what are its semantics? $ whoami | >out tr 'a 'b' Andreas. -- Andreas Schwab, SUSE Labs, sch...@suse.de GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE 1748 E4D4 88E3 0EEA B9D7 "And now for something completely different."
RE: Feature: disallow | >
Thanks. So 'whoami | >out' working is a natural consequence of the fact that '>out' is a command by itself. IMHO it would have been better for the Unix shell to forbid that, and require ': >out' if you really do want to run the null command and redirect its output, but it's too late now. -- Ed Avis __ This email has been scanned by the Symantec Email Security.cloud service. For more information please visit http://www.symanteccloud.com __
Re: Feature: disallow | >
Ed Avis wrote: > Thanks. So 'whoami | >out' working is a natural consequence of the > fact that '>out' is a command by itself. It is a natural consequence of parsing redirections before parsing commands. The redirection happens before the command execution and an empty command is valid. That redirections happen in an earlier pass across the command line is why they can occur anywhere in the command. By convention we put rediections at the end. But they can also validly occur first (as the previous example showed) or in the middle. > IMHO it would have been better for the Unix shell to forbid that, > and require ': >out' if you really do want to run the null command > and redirect its output, but it's too late now. If it did then it would be incompatible with all of the rest that allow it. As I recall one of the obscure fringe Unix lookalike systems did exactly that in their shell. I forget details now. But because of that some of us do always use ":" instead of an empty redirection simply due to it. Bob
