Shell scripting: -z and -n options with if

Question

I have a shell script where we have following lines if [ -z "$xyz" ] and if [ -n "$abc" ], but I am not sure what their purpose is. Can anyone please explain?

Answer 1

You can find a very nice reference for bash's operators here. If you are using a different shell, just search for <my shell> operators and you will find everything you need. In your particular case, you are using:

-n
   string is not null.

-z
  string is null, that is, has zero length

To illustrate:

$ foo="bar";
$ [ -n "$foo" ] && echo "foo is not null"
foo is not null
$ [ -z "$foo" ] && echo "foo is null"
$ foo="";
$ [ -n "$foo" ] && echo "foo is not null"
$ [ -z "$foo" ] && echo "foo is null"
foo is null

Answer 2

To extend terdon's answer, I found that Unix / Linux - Shell Basic Operators on Tutorials Point also includes file-related operators (as well as other useful ones).

-b file     Checks if file is a block special file; if yes, then the condition becomes true.    [ -b $file ] is false.
-c file     Checks if file is a character special file; if yes, then the condition becomes true.    [ -c $file ] is false.
-d file     Checks if file is a directory; if yes, then the condition becomes true.     [ -d $file ] is not true.
-f file     Checks if file is an ordinary file as opposed to a directory or special file; if yes, then the condition becomes true.  [ -f $file ] is true.
-g file     Checks if file has its set group ID (SGID) bit set; if yes, then the condition becomes true.    [ -g $file ] is false.
-k file     Checks if file has its sticky bit set; if yes, then the condition becomes true.     [ -k $file ] is false.
-p file     Checks if file is a named pipe; if yes, then the condition becomes true.    [ -p $file ] is false.
-t file     Checks if file descriptor is open and associated with a terminal; if yes, then the condition becomes true.  [ -t $file ] is false.
-u file     Checks if file has its Set User ID (SUID) bit set; if yes, then the condition becomes true.     [ -u $file ] is false.
-r file     Checks if file is readable; if yes, then the condition becomes true.    [ -r $file ] is true.
-w file     Checks if file is writable; if yes, then the condition becomes true.    [ -w $file ] is true.
-x file     Checks if file is executable; if yes, then the condition becomes true.  [ -x $file ] is true.
-s file     Checks if file has size greater than 0; if yes, then condition becomes true.    [ -s $file ] is true.
-e file     Checks if file exists; is true even if file is a directory but exists.  [ -e $file ] is true.

Answer 3

man test or man [ will give you all the options to test command. In this case, -n is testing to see if the content of $abc has a non-zero length and -z is testing to see if the content of $xyz is a zero-length string.