Redirecting or piping 'somecommand --help'.

stf92 · 10-31-2010, 06:29 AM

kernel 2.6, slackware 12.0
mkisofs 2.01

Hi:
If I do 'ls --help|more' all's well. 'mkisofs --help' outputs its help screen, and I can use Shift+PgUp/PgDn to scroll through it. But I can neither pipe it to more or to less, nor redirect it to some file. more is simply ignored. Less, gets into less but only the last screenful is seen. Redirection, i.e. 'mkisofs --help>john.txt' produces an empty file (size= 0). And the same happens with some other commands, besides mkisofs. Is there a way around this? Thanks is advance.

David the H. · 10-31-2010, 07:07 AM

The help text is being printed to stderr instead of stdout. You need to use I/O redirection in order to access it directly.

To redirect stderr to stdout in the shell:

Code:

mkisofs --help 2>&1 | less

To redirect stderr to a file:

Code:

mkisofs --help 2>filename

There's more on I/O redirection here:
http://tldp.org/LDP/abs/html/io-redirection.html

stf92 · 10-31-2010, 01:56 PM

Thank you for your kind and useful reply. In the meantime, I remembered I had seen a command named script and, using it, I sent the help output to a file. But your solution is more economical. Above all, to know that the output was being sent to stderr was a great thing. Regards.