-z option inside if condition in shell script

问题

Can someone post me the significance of -z option while writing if condition using shell scripts.

Let me paste the exact condition what I am looking at

if [[ "x$do_clean_flag" = "x-clean" && -z "$show_me_flag" && "$verify" = true ]]

回答1:

From "help test":

-z STRING      True if string is empty.

回答2:

-z checks whether $show_me_flag is defined

check out the manual of test(1)

回答3:

-z (something) means if (something) is NULL then return true

http://unixhelp.ed.ac.uk/CGI/man-cgi?test

suggests that zero length returns true as well, I'd suggest doing "man test" on your machine to check the exact wording on your system.

来源：https://stackoverflow.com/questions/5594039/z-option-inside-if-condition-in-shell-script