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UGU: Unix Guru Universe - Unix Tip #4433- May 1, 2013 - Home : Help : Today's Tip
Unix Tip #4433- May 1, 2013

TEST TROUBLES IF STRING IS EMPTY


(1) bad ex.
if [ `echo "$OPTARG" | sed '/^[0-9][0-9]*$/!d` = "" ]

(2) fixed ex.
if [ `echo "$OPTARG" | sed '/^[0-9][0-9]*$/ s//X/'`= "X" ]

(3) another fixed ex. (Bourne Shell, ksh)
if [ -n "`echo "$OPTARG" | sed '/^[0-9][0-9]*$/!d'`" ]

(1) This produces an easy
to overlook error. On
success and the lack of
double quotes on the left
side cause test to think
there is no parameter,
producing an error (1).
This is because the result
is an empty string without
quotes and test doesn't
know that it is dealing
with a string, no parameter
seen.

Adding a character to the
beginning of each string is
a trick to fix it (2). The
test command sees a string
and simply compares as normal,
passing over the pair of
initial and equal characters.

Another fix (3) is to surround
the empty/not empty string with
double quotes. Test will see
the empty string and things
work as normal. This (3) works
in Bourne Shell and ksh but NOT
in csh.

Simple ex.
$ foo=""
$ test -n $foo # fails
$ test -n `echo $foo` # fails
$ test -z `echo x$foo` # works, might throw off the logic
$ test -n "`echo $foo`" # works in sh & ksh


This tip generously supported by: bigoldbulldog@hotmail.com

NOTE: All tips provided are USE AT YOUR OWN RISK. Tips are submitted by various unix admins around the globe. UGU suggest you read and test each tip in a non-volitile environment before placing into production.


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