Check bash array element for whitespace or empty

darkangel29 · 08-06-2013, 03:05 PM

I have a script that separates the input string into an array my problem is that I want to check determine if the array element is whitepace or empty do something like a substitution. This is what I have and it works for any value except whitespace or empty.

Code:

echo -n "Enter string: "
read s

i=0
while [ $i -lt ${#s} ]; do y[$i]=${s:$i:1};  i=$((i+1));done

for (( k=0; k < $length; k++ ))
do
 if [ ${y[$k]} == " " ]  
  then
   y[$k]=32
 fi
done

But it doesn't substitute the whitespace and I have tried many other different combinations. If I try another value it works.

Code:

for (( k=0; k < $length; k++ ))
do
 if [ ${y[$k]} == "a" ]  
  then
   y[$k]=32
 fi

Thanks for the help!

vmccord · 08-06-2013, 03:22 PM

Your if statement is looking for a space, but you mention looking for empty which I think of as null. in bash it would be

Code:

 if [ -z "${y[$k]}" ]

Firerat · 08-06-2013, 03:27 PM

Code:

if [ "${y[$k]}" == " " ]

you need quotes, else shell sees just

Code:

if [   == " " ]

darkangel29 · 08-06-2013, 03:33 PM

Thanks!! It was the quotes

Quote:

if [ "${y[$k]}" == " " ]

did the trick

Firerat · 08-06-2013, 03:44 PM

one other thing

in your script you have undeclared $length

I assume you already declared that length somewhere else

Code:

for (( k=0; k < ${#s}; k++ ))

is safer

only mention it, as it could be a 'bug' for you, ( since you have a new s, with potential for new length )

but I guess you were just using read s for purpose of example

konsolebox · 08-06-2013, 11:02 PM

You could also check if an array element is empty but is still set through generation of indices:

Code:

function has_element {
    IFS=' ' eval "[[ \" \${!$1[*]} \" == *\" $2 \"* ]]"
}

A=([1]="")

has_element A 1 && echo a ## messages a

To check if a string has one or more spaces:

Code:

shopt -s extglob
if [[ $VAR == +([[:blank:]]) ]]; then ...
# or
if [[ $VAR =~ ^[[:blank:]]+$ ]]; then ...

To check if a string is empty, or has one or more spaces, use *:

Code:

shopt -s extglob
if [[ $VAR == *([[:blank:]]) ]]; then ...
# or
if [[ $VAR =~ ^[[:blank:]]*$ ]]; then ...

You could use [:space:] instead of [:blank:] but I don't know how they differ. Or should I say I forgot. But I had a reason to use [:blank:] over it.

SAbhi · 08-07-2013, 12:31 AM

Quote:

You could use [:space:] instead of [:blank:] but I don't know how they differ. Or should I say I forgot. But I had a reason to use [:blank:] over it.

that's pretty easy:
:blank: > will work when there is no value supplies not even a space.
you can write simple program and test with using "" and " ";

konsolebox · 08-07-2013, 01:35 AM

Quote:

Originally Posted by SAbhi

that's pretty easy:
:blank: > will work when there is no value supplies not even a space.

Where did you get that from? [:blank:] despite the name doesn't represent empty values.

Code:

shopt -s extglob
[[ "" == +([[:blank:]]) ]] || echo "No it doesn't."

Anyway looking at gawk's manual, these are the differences:

Code:

[:blank:]  Space or tab characters.
[:space:]  Space characters (such as space, tab, and formfeed, to name a few).

SAbhi · 08-07-2013, 02:12 AM

AWWW sorry for this, its my mistake, what I was thinking when I posted that? :P

I by mistake compared it with ""