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FreeBSD 11.0 - man page for look (freebsd section 1)

LOOK(1) 						    BSD General Commands Manual 						   LOOK(1)

look -- display lines beginning with a given string
look [-df] [-t termchar] string [file ...]
The look utility displays any lines in file which contain string as a prefix. As look performs a binary search, the lines in file must be sorted. If file is not specified, the file /usr/share/dict/words is used, only alphanumeric characters are compared and the case of alphabetic char- acters is ignored. The following options are available: -d, --alphanum Dictionary character set and order, i.e., only alphanumeric characters are compared. -f, --ignore-case Ignore the case of alphabetic characters. -t, --terminate termchar Specify a string termination character, i.e., only the characters in string up to and including the first occurrence of termchar are compared.
The LANG, LC_ALL and LC_CTYPE environment variables affect the execution of the look utility. Their effect is described in environ(7).
/usr/share/dict/words the dictionary
The look utility exits 0 if one or more lines were found and displayed, 1 if no lines were found, and >1 if an error occurred.
The original manual page stated that tabs and blank characters participated in comparisons when the -d option was specified. This was incor- rect and the current man page matches the historic implementation. The -a and --alternative flags are ignored for compability.
grep(1), sort(1)
A look utility appeared in Version 7 AT&T UNIX.
Lines are not compared according to the current locale's collating order. Input files must be sorted with LC_COLLATE set to 'C'.
July 17, 2004 BSD