Filetrans Function (The GNU Awk User’s Guide)
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10.3.1 Noting Data file Boundaries
The BEGIN
and END
rules are each executed exactly once, at the beginning and end of your awk
program, respectively (see section The BEGIN and END Special Patterns). We (the gawk
authors) once had a user who mistakenly thought that the BEGIN
rules were executed at the beginning of each data file and the END
rules were executed at the end of each data file.
When informed that this was not the case, the user requested that we add new special patterns to gawk
, named BEGIN_FILE
and END_FILE
, that would have the desired behavior. He even supplied us the code to do so.
Adding these special patterns to gawk
wasn’t necessary; the job can be done cleanly in awk
itself, as illustrated by the following library program. It arranges to call two user-supplied functions, beginfile()
and endfile()
, at the beginning and end of each data file. Besides solving the problem in only nine(!) lines of code, it does so portably; this works with any implementation of awk
:
# transfile.awk # # Give the user a hook for filename transitions # # The user must supply functions beginfile() and endfile() # that each take the name of the file being started or # finished, respectively. FILENAME != _oldfilename { if (_oldfilename != "") endfile(_oldfilename) _oldfilename = FILENAME beginfile(FILENAME) } END { endfile(FILENAME) }
This file must be loaded before the user’s “main” program, so that the rule it supplies is executed first.
This rule relies on awk
’s FILENAME
variable, which automatically changes for each new data file. The current file name is saved in a private variable, _oldfilename
. If FILENAME
does not equal _oldfilename
, then a new data file is being processed and it is necessary to call endfile()
for the old file. Because endfile()
should only be called if a file has been processed, the program first checks to make sure that _oldfilename
is not the null string. The program then assigns the current file name to _oldfilename
and calls beginfile()
for the file. Because, like all awk
variables, _oldfilename
is initialized to the null string, this rule executes correctly even for the first data file.
The program also supplies an END
rule to do the final processing for the last file. Because this END
rule comes before any END
rules supplied in the “main” program, endfile()
is called first. Once again, the value of multiple BEGIN
and END
rules should be clear.
If the same data file occurs twice in a row on the command line, then endfile()
and beginfile()
are not executed at the end of the first pass and at the beginning of the second pass. The following version solves the problem:
# ftrans.awk --- handle datafile transitions # # user supplies beginfile() and endfile() functions FNR == 1 { if (_filename_ != "") endfile(_filename_) _filename_ = FILENAME beginfile(FILENAME) } END { endfile(_filename_) }
Counting Things shows how this library function can be used and how it simplifies writing the main program.
So Why Does You are probably wondering, if Good question. Normally, if |
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