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Some of these techniques will require non-ancient versions of awk.
The most portable way to test for the existence of a file is to simply try and read from the file.
function exists(file, dummy, ret)
{
ret=0;
if ( (getline dummy < file) >=0 )
{
# file exists (possibly empty) and can be read
ret = 1;
close(file);
}
return ret;
}
[ I've read reports that earlier versions of mawk would write to stderr as well as getline returning <0 -- is this still true? ]
On Unix, you can probably use the `test' utility
if (system("test -r " file) == 0)
# file is readable
else
# file is not readable
awk automatically reads multiple files (under Unix at least) -- use something like:
awk '/^#include/ {print $2}' *.c *.hthe file name is stored in the built-in variable FILENAME:
awk '/^#include/ {print FILENAME,$2}' *.c *.hYou can open files dynamically using `getline', `close', and `print EXPR > FILENAME', like:
# assumes input file has at least 1 line, output file writeable
function double(infilename,outfilename, aline)
{
while ( (getline aline < infilename) >0 )
print(aline aline) > outfilename;
close(infilename);
close(outilename);
}
you can tell if awk is parsing the first file given on the command line using FILENAME, thusly:
BEGIN { rulesfile="" }
rulesfile == "" { rulesfile = FILENAME; }
FILENAME == rulesfile { build_rule($0); }
FILENAME != rulesfile { apply_rule($0); }
Example:
Suppose you have a text-line "database" and you want to make some batch changes to it, by replacing some old lines with new lines.
BEGIN { rulesfile="" }
rulesfile == "" { rulesfile = FILENAME; }
rulesfile == FILENAME { replace[$1] = $0; }
rulesfile != FILENAME \
{
if ($1 in replace)
print replace[$1];
else
print;
}
Another way, using ARGV:
(FILENAME == ARGV[1]) { replace[$1] = $0; next }
($1 in replace) { print replace[$1]; next }
{ print }
You can use `-v rulesfile=filename' to process a file differently, like you would any other variable, and then use a `getline' loop (and `close') in your BEGIN statement.
BEGIN \
{
if (rulesfile=="")
{
print "must use -v rulesfile=filename";
exit(1);
}
while ( (getline < rulesfile) >0 )
replace[$1]=$0;
close(rulesfile);
}
{
if ($1 in replace)
print replace[$1];
else
print;
}
