Unix Tip: TEST TROUBLES IF STRING IS EMPTY

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UNIX HOT TIP

Unix Tip 3417 - June 18, 2011

http://www.ugu.com/sui/ugu/show?tip.today

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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

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