05-29-2010
what happens is:
the shell reads the command line, substitutes parameters
then it will spot the redirect which says
"truncate the file and send ouput to it".
so it truncates the file.
only then does it run the command, but by then the
file's been trashed.
see?
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UNIQ(1) General Commands Manual UNIQ(1)
NAME
uniq - report repeated lines in a file
SYNOPSIS
uniq [ -udc [ +n ] [ -n ] ] [ input [ output ] ]
DESCRIPTION
Uniq reads the input file comparing adjacent lines. In the normal case, the second and succeeding copies of repeated lines are removed;
the remainder is written on the output file. Note that repeated lines must be adjacent in order to be found; see sort(1). If the -u flag
is used, just the lines that are not repeated in the original file are output. The -d option specifies that one copy of just the repeated
lines is to be written. The normal mode output is the union of the -u and -d mode outputs.
The -c option supersedes -u and -d and generates an output report in default style but with each line preceded by a count of the number of
times it occurred.
The n arguments specify skipping an initial portion of each line in the comparison:
-n The first n fields together with any blanks before each are ignored. A field is defined as a string of non-space, non-tab charac-
ters separated by tabs and spaces from its neighbors.
+n The first n characters are ignored. Fields are skipped before characters.
SEE ALSO
sort(1), comm(1)
UNIQ(1)