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gawk: Getline

 4.10 Explicit Input with 'getline'
 So far we have been getting our input data from 'awk''s main input
 stream--either the standard input (usually your keyboard, sometimes the
 output from another program) or the files specified on the command line.
 The 'awk' language has a special built-in command called 'getline' that
 can be used to read input under your explicit control.
    The 'getline' command is used in several different ways and should
 _not_ be used by beginners.  The examples that follow the explanation of
 the 'getline' command include material that has not been covered yet.
 Therefore, come back and study the 'getline' command _after_ you have
 reviewed the rest of this Info file and have a good knowledge of how
 'awk' works.
    The 'getline' command returns 1 if it finds a record and 0 if it
 encounters the end of the file.  If there is some error in getting a
 record, such as a file that cannot be opened, then 'getline' returns -1.
 In this case, 'gawk' sets the variable 'ERRNO' to a string describing
 the error that occurred.
    If 'ERRNO' indicates that the I/O operation may be retried, and
 'PROCINFO["INPUT", "RETRY"]' is set, then 'getline' returns -2 instead
 of -1, and further calls to 'getline' may be attempted.  ⇒Retrying
 Input for further information about this feature.
    In the following examples, COMMAND stands for a string value that
 represents a shell command.
      NOTE: When '--sandbox' is specified (⇒Options), reading
      lines from files, pipes, and coprocesses is disabled.


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