IF $ has value then?

UNIX for Beginners Q & A UNIX for Dummies Questions & Answers
1

Hello,

I was wondering if there is any way to declare something like this pseudo code:

If [$x HAS A VALUE OR IS NOT EMPTY] ;then?

how to write to check if that $x has a value or anything?

2

test if "$x" is not empty

if [ -n "$x" ]; then
...
else
...
fi

test if "$x" is empty (has a length of zero) :

if [ -z "$x" ]; then
...
else
...
fi

or (if $x has a length of zero)

if [ ${#x} -eq 0 ]; then
...
else
...
fi

# a=
# [[ ${#a} -eq 0 ]] && echo yes || echo no
yes
# a=x
# [[ ${#a} -eq 0 ]] && echo yes || echo no
no
# a=123
# [[ ${#a} -eq 0 ]] && echo yes || echo no
no
# a=0
# [[ ${#a} -eq 0 ]] && echo yes || echo no
no

You could also consider the following :

[[ -z "$a" ]] && echo Empty || echo NotEmpty
[[ ! -z "$a" ]] && echo NotEmpty || echo Empty
[[ -n "$a" ]] && echo NotEmpty || echo Empty

In fact the doublequote enclosing the $a are not necessary in [[ ]] (ksh and bash)

3

Thank you, it works :}

4

Here is a tricky example here is the code :

mytst(){
a=${1-whatever}
echo $a
 }

Now let's run the code ...
If you understand every result, then you got how it works ...

# mytst
whatever
# mytst 1
1
# mytst 2
2
# mytst ""

# mytst
whatever
# mytst If no arg is given set a to whatever
If
# mytst "If no arg is given set a to whatever"
If no arg is given set a to whatever
# mytst "          "

# mytst ""

# mytst
whatever
#

By the way, you should carefully read this (you may find the "Syntax of String Operator" section useful.)