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sed: Numeric Addresses

 
 4.2 Selecting lines by numbers
 ==============================
 
 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 STEPth 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
 

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