[SOLVED] troubles with bash script tests "-z" and "-n"

SaintDanBert · 11-04-2010, 06:25 PM

(blush) I should be able to sort this out, but I'm stumped.

Here are some commands and the results:

Code:

stuff="xxx xxx"      # the blank is important
echo $stuff
xxx xxx
if [ -z stuff ]; then echo nothing; fi     # no output
if [ -n stuff ]; then echo non-zero; fi    # output
non-zero

Code:

stuff=""
echo "yyy $stuff yyy"     # output has two blanks -- check
yyy  yyy
if [ -z stuff ]; then echo nothing; fi    # no output ???
if [ -n stuff ]; then echo non-zero; fi   # output ???
non-zero

The bash man-page says:

-z string -- true if length of string is zero
-n string -- true if length of string is non-zero

I thought that stuff="" creates a string of length==0.
There is something obvious here that I am obviously missing.

I'll take my lumps, but I left my flame-suit at the home.
~~~ 0;-Dan

ZaSter · 11-04-2010, 06:40 PM

After setting the variable "stuff", you must from then on refer to it as $stuff. You are simply missing the dollar sign in your tests of "stuff".

rupertwh · 11-04-2010, 06:44 PM

This missing dollar is one thing.
Also, you must enclose the variable in quotes for the test, otherwise empty vars will not work as you might expect:

Code:

if [ -z "$stuff" ];

peonuser · 11-04-2010, 07:00 PM

You might look at this page.

http://tldp.org/LDP/abs/html/comparison-ops.html

SaintDanBert · 11-05-2010, 06:13 PM

I knew that I was [not] doing something silly.

Thanks to all.

Thanks, also, for avoiding the call to rub my nose in this newbie mistake.

Cheers,
~~~ 0;-Dan

SaintDanBert · 11-05-2010, 06:17 PM

Since the solution involved quoting within scripts, I offer the following from the man bash manual page:

Code:

Enclosing  characters  in  single  quotes preserves the literal value of each character within the quotes.  A single quote may not occur between single quotes, even when preceded by a backslash.

Enclosing characters in double quotes preserves the literal value of
all  characters  within  the quotes,  with the exception of $, `, \, 
and, when history expansion is enabled, !.  The characters $ and `
retain their special meaning within double quotes.   The  backslash
retains  its  special meaning only when followed by one of the 
following characters: $, `, ", \, or <newline>.  A double quote may
be quoted within double quotes by preceding it with a backslash.   
If  enabled,  history expansion  will be performed unless an !  appearing in double quotes is escaped using a backslash.

The backslash preceding the !  is not removed.

Cheers,
~~~ 0;-Dan

ZaSter · 04-09-2012, 09:13 PM

Your "if" tests are missing the "$" on your variable names.

David the H. · 04-10-2012, 09:26 AM

@ZaSter

Please don't re-open old threads for trivial purposes. Especially since you (!) already said exactly the same thing two and a half years ago, in the second post.

This is a classic case of bash pitfall #4, by the way.

http://mywiki.wooledge.org/BashPitfalls