Info Catalog gawk: BEGIN/END gawk: Pattern Overview gawk: Empty

gawk: BEGINFILE/ENDFILE

 
 7.1.5 The 'BEGINFILE' and 'ENDFILE' Special Patterns
 ----------------------------------------------------
 
 This minor node describes a 'gawk'-specific feature.
 
    Two special kinds of rule, 'BEGINFILE' and 'ENDFILE', give you
 "hooks" into 'gawk''s command-line file processing loop.  As with the
 'BEGIN' and 'END' rules (BEGIN/END), all 'BEGINFILE' rules in a
 program are merged, in the order they are read by 'gawk', and all
 'ENDFILE' rules are merged as well.
 
    The body of the 'BEGINFILE' rules is executed just before 'gawk'
 reads the first record from a file.  'FILENAME' is set to the name of
 the current file, and 'FNR' is set to zero.
 
    The 'BEGINFILE' rule provides you the opportunity to accomplish two
 tasks that would otherwise be difficult or impossible to perform:
 
    * You can test if the file is readable.  Normally, it is a fatal
      error if a file named on the command line cannot be opened for
      reading.  However, you can bypass the fatal error and move on to
      the next file on the command line.
 
      You do this by checking if the 'ERRNO' variable is not the empty
      string; if so, then 'gawk' was not able to open the file.  In this
      case, your program can execute the 'nextfile' statement (
      Nextfile Statement).  This causes 'gawk' to skip the file
      entirely.  Otherwise, 'gawk' exits with the usual fatal error.
 
    * If you have written extensions that modify the record handling (by
      inserting an "input parser"; Input Parsers), you can invoke
      them at this point, before 'gawk' has started processing the file.
      (This is a _very_ advanced feature, currently used only by the
      'gawkextlib' project
      (https://sourceforge.net/projects/gawkextlib).)
 
    The 'ENDFILE' rule is called when 'gawk' has finished processing the
 last record in an input file.  For the last input file, it will be
 called before any 'END' rules.  The 'ENDFILE' rule is executed even for
 empty input files.
 
    Normally, when an error occurs when reading input in the normal
 input-processing loop, the error is fatal.  However, if an 'ENDFILE'
 rule is present, the error becomes non-fatal, and instead 'ERRNO' is
 set.  This makes it possible to catch and process I/O errors at the
 level of the 'awk' program.
 
    The 'next' statement (Next Statement) is not allowed inside
 either a 'BEGINFILE' or an 'ENDFILE' rule.  The 'nextfile' statement is
 allowed only inside a 'BEGINFILE' rule, not inside an 'ENDFILE' rule.
 
    The 'getline' statement (Getline) is restricted inside both
 'BEGINFILE' and 'ENDFILE': only redirected forms of 'getline' are
 allowed.
 
    'BEGINFILE' and 'ENDFILE' are 'gawk' extensions.  In most other 'awk'
 implementations, or if 'gawk' is in compatibility mode (
 Options), they are not special.
 

Info Catalog gawk: BEGIN/END gawk: Pattern Overview gawk: Empty