Gawk has a too-large number of extensions to POSIX awk. They are
described in this section. All the extensions described here can
be disabled by invoking gawk with the --traditional or --posix
options.
The following features of gawk are not available in POSIX awk.
• No path search is performed for files named via the -f
option. Therefore the AWKPATH environment variable is not
special.
• There is no facility for doing file inclusion (gawk's
@include mechanism).
• There is no facility for dynamically adding new functions
written in C (gawk's @load mechanism).
• The \x escape sequence.
• The ability to continue lines after ? and :.
• Octal and hexadecimal constants in AWK programs.
• The ARGIND, BINMODE, ERRNO, LINT, PREC, ROUNDMODE, RT and
TEXTDOMAIN variables are not special.
• The IGNORECASE variable and its side-effects are not
available.
• The FIELDWIDTHS variable and fixed-width field splitting.
• The FPAT variable and field splitting based on field
values.
• The FUNCTAB, SYMTAB, and PROCINFO arrays are not
available.
• The use of RS as a regular expression.
• The special file names available for I/O redirection are
not recognized.
• The |& operator for creating coprocesses.
• The BEGINFILE and ENDFILE special patterns are not
available.
• The ability to split out individual characters using the
null string as the value of FS, and as the third argument
to split().
• An optional fourth argument to split() to receive the
separator texts.
• The optional second argument to the close() function.
• The optional third argument to the match() function.
• The ability to use positional specifiers with printf and
sprintf().
• The ability to pass an array to length().
• The and(), asort(), asorti(), bindtextdomain(), compl(),
dcgettext(), dcngettext(), gensub(), lshift(), mktime(),
or(), patsplit(), rshift(), strftime(), strtonum(),
systime() and xor() functions.
• Localizable strings.
• Non-fatal I/O.
• Retryable I/O.
The AWK book does not define the return value of the close()
function. Gawk's close() returns the value from fclose(3), or
pclose(3), when closing an output file or pipe, respectively. It
returns the process's exit status when closing an input pipe.
The return value is -1 if the named file, pipe or coprocess was
not opened with a redirection.
When gawk is invoked with the --traditional option, if the fs
argument to the -F option is 't', then FS is set to the tab
character. Note that typing gawk -F\t ... simply causes the
shell to quote the 't,' and does not pass '\t' to the -F option.
Since this is a rather ugly special case, it is not the default
behavior. This behavior also does not occur if --posix has been
specified. To really get a tab character as the field separator,
it is best to use single quotes: gawk -F'\t' ....
