Shell Programming and Scripting

hi,

i have stuff like

Code:

if [[ x$ip = x127.0.0.1 ]]; then

what does the x mean?

also, what does -z variable mean in an if statement>

thanks

x doesn't have special meaning it is here just in case $ip is empty

-z means if empty (z stand for a zero length)

sorry, i am confused, if ip is empty and we did the check, it would fail anyway, so why would we worry about this x stuff?

thanks

remove the x , make a test with an empty ip and see what happen ...



---------- Post updated at 10:09 PM ---------- Previous update was at 10:07 PM ----------




if $ip is empty :

Code:

[[ x$ip = x127.0.0.1 ]]
[[ x = x127.0.0.1 ]] ===> false

now if $ip is empty without x :

Code:

[[ $ip = 127.0.0.1 ]]
[[  = 127.0.0.1 ]] ===> Error

Note that this would not happen using double quote

Code:

[[ "$ip" = "127.0.0.1" ]]
[[ "" = "127.0.0.1" ]] ===> false

Ok course these behaviours may depends on your system OS and SHELL implementation, but this is what "could" be expected

Too lazy to use ", safe if $ip is blank but not if it is "x x"!

Code:

if [[ "$ip" = "127.0.0.1" ]]; then

-z is empty string, see man test.

great answers guys, thanks

if you remove the x, and $ip is empty, you get

Code:

if [[  = x127.0.0.1 ]]; then

which leads to an error.
the same with x and empty $ip:

Code:

if [[ x = x127.0.0.1 ]]; then

no errors here.

You can as well enclose $ip with double-quotes so if $ip is empty, you still have an argument to the left of the =:

Code:

if [[ "" = x127.0.0.1 ]]; then

or you can use that -z, but it only test for zero string, you can't use this if you need to check different IPs:

Code:

if [[ -z $ip ]]; then

says "if string $ip is zero-length, then..."