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CentOS 7.0 - man page for look (centos section 1)

LOOK(1) 							   User Commands							   LOOK(1)

look - display lines beginning with a given string
look [options] string [file]
The look utility displays any lines in file which contain string. As look performs a binary search, the lines in file must be sorted (where sort(1) got the same options -d and/or -f that look is invoked with). If file is not specified, the file /usr/share/dict/words is used, only alphanumeric characters are compared and the case of alphabetic characters is ignored.
-a, --alternative Use the alternative dictionary file. -d, --alphanum Use normal dictionary character set and order, i.e. only alphanumeric characters are compared. (This is on by default if no file is specified.) -f, --ignore-case Ignore the case of alphabetic characters. (This is on by default if no file is specified.) -t, --terminate character Specify a string termination character, i.e. only the characters in string up to and including the first occurrence of character are compared. -h, --help Display help text and exit. -V, --version Output version information and exit. 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.
sort -d /etc/passwd -o /tmp/look.dict look -t: root:foobar /tmp/look.dict
/usr/share/dict/words the dictionary /usr/share/dict/web2 the alternative dictionary
grep(1), sort(1)
The original manual page stated that tabs and blank characters participated in comparisons when the alphanum option was specified. This was incorrect, and the current man page matches the historic implementation.
The look utility appeared in Version 7 AT&T Unix.
The look command is part of the util-linux package and is available from util-linux June 2011 LOOK(1)