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Addresses in a sed
script can be in any of the following forms:
number
Specifying a line number will match only that line in the input.
(Note that sed
counts lines continuously across all input files
unless -i
or -s
options are specified.)
$
This address matches the last line of the last file of input, or
the last line of each file when the -i
or -s
options
are specified.
first~step
This GNU extension matches every step
th line
starting with line first
.
In particular, lines will be selected when there exists
a non-negative n
such that the current line-number equals
first
+ (n
* step
).
Thus, one would use 1~2
to select the odd-numbered lines and
0~2
for even-numbered lines;
to pick every third line starting with the second, ‘2~3
’ would be used;
to pick every fifth line starting with the tenth, use ‘10~5
’;
and ‘50~0
’ is just an obscure way of saying 50
.
The following commands demonstrate the step address usage:
$ seq 10 | sed -n '0~4p' 4 8 $ seq 10 | sed -n '1~3p' 1 4 7 10