Explain What This Awk Command Does

Question

I have a portion of the VoIPBL update script.

cat /tmp/voipbl.txt | awk '{ print "if [ ! -z \""$1"\" -a \""$1"\"  != \"#\" ]; then /usr/sbin/ipset  -A voipbl_temp \""$1"\" ;fi;"}'

It creates lines like this:

if [ ! -z "1.2.3.4/32" -a "1.2.3.4/32"  != "#" ]; then  /usr/sbin/ipset -A voipbl_temp "1.2.3.4/32" ;fi; | sh

I understand the second part is adding a line from the file (IP) to an ipset, but what is the test condition doing? What does it mean when you have -z and -a in the same set of brackets? And what does the not equal to "#" part mean?

Answer 1

It's reading presumably IPs from the /tmp/voipbl.txt file and then creating a shell if construct based on them. The $1 parameter from the awk script refers to the item in the first column on each line so the string in place of 1.2.3.4/32 will be that value in each iteration.

if [ ! -z "1.2.3.4/32" -a "1.2.3.4/32"  != "#" ]; then

! -z "1.2.3.4/32" - Is checking that "1.2.3.4/32" is not null. -z checks if the string is zero but the ! negates that. This could and probably should be replaced with -n which just checks for not-null

"1.2.3.4/32" != "#" - Is checking that "1.2.3.4/32" is not equal to # which I guess is something that could appear in your file.

The -a operator means and. So it's checking that the string is not null AND that it does not equal #.

If these conditions are met, the following will be executed:

/usr/sbin/ipset -A voipbl_temp "1.2.3.4/32"

To understand this better, try running the awk command on a file with the following contents:

1.2.3.4/32

# 3.4.5.6/32

If you now run your awk command on that file, you get:

$ awk '{ print "if [ ! -z \""$1"\" -a \""$1"\"  != \"#\" ]; then /usr/sbin/ipset  -A voipbl_temp \""$1"\" ;fi;"}' file
if [ ! -z "1.2.3.4/32" -a "1.2.3.4/32"  != "#" ]; then /usr/sbin/ipset  -A voipbl_temp "1.2.3.4/32" ;fi;
if [ ! -z "" -a ""  != "#" ]; then /usr/sbin/ipset  -A voipbl_temp "" ;fi;
if [ ! -z "#" -a "#"  != "#" ]; then /usr/sbin/ipset  -A voipbl_temp "#" ;fi;

The tests ensure that the /usr/sbin/ipset will only be run on the first line, with 1.2.3.4/32 and not on the other two.