Info Catalog gawk: Getline/Variable/File gawk: Getline gawk: Getline/Variable/Pipe

gawk: Getline/Pipe

 
 4.10.5 Using 'getline' from a Pipe
 ----------------------------------
 
      Omniscience has much to recommend it.  Failing that, attention to
      details would be useful.
                          -- _Brian Kernighan_
 
    The output of a command can also be piped into 'getline', using
 'COMMAND | getline'.  In this case, the string COMMAND is run as a shell
 command and its output is piped into 'awk' to be used as input.  This
 form of 'getline' reads one record at a time from the pipe.  For
 example, the following program copies its input to its output, except
 for lines that begin with '@execute', which are replaced by the output
 produced by running the rest of the line as a shell command:
 
      {
           if ($1 == "@execute") {
                tmp = substr($0, 10)        # Remove "@execute"
                while ((tmp | getline) > 0)
                     print
                close(tmp)
           } else
                print
      }
 
 The 'close()' function is called to ensure that if two identical
 '@execute' lines appear in the input, the command is run for each one.
 Close Files And Pipes.  Given the input:
 
      foo
      bar
      baz
      @execute who
      bletch
 
 the program might produce:
 
      foo
      bar
      baz
      arnold     ttyv0   Jul 13 14:22
      miriam     ttyp0   Jul 13 14:23     (murphy:0)
      bill       ttyp1   Jul 13 14:23     (murphy:0)
      bletch
 
 Notice that this program ran the command 'who' and printed the result.
 (If you try this program yourself, you will of course get different
 results, depending upon who is logged in on your system.)
 
    This variation of 'getline' splits the record into fields, sets the
 value of 'NF', and recomputes the value of '$0'.  The values of 'NR' and
 'FNR' are not changed.  'RT' is set.
 
    According to POSIX, 'EXPRESSION | getline' is ambiguous if EXPRESSION
 contains unparenthesized operators other than '$'--for example, '"echo "
 "date" | getline' is ambiguous because the concatenation operator is not
 parenthesized.  You should write it as '("echo " "date") | getline' if
 you want your program to be portable to all 'awk' implementations.
 
      NOTE: Unfortunately, 'gawk' has not been consistent in its
      treatment of a construct like '"echo " "date" | getline'.  Most
      versions, including the current version, treat it at as '("echo "
      "date") | getline'.  (This is also how BWK 'awk' behaves.)  Some
      versions instead treat it as '"echo " ("date" | getline)'.  (This
      is how 'mawk' behaves.)  In short, _always_ use explicit
      parentheses, and then you won't have to worry.
 

Info Catalog gawk: Getline/Variable/File gawk: Getline gawk: Getline/Variable/Pipe