How to check if variable (usually text) Is not defined or empty, if variable is empty uses other variable [duplicate]

Question

Script A imports all variables from B script; the problem is script B, I do not know if exist or is empty the variable as I need. Case not exist variable in B, then I use my variable in script A. case it exist in A use my variable B; or it exist in A and B then use for default variable in B.

Example: In this case the variable Comment3 in my script "B" is empty...

script B.sh

#!/bin/sh
Name='<b>Variety</b>'
Comment='<span size="xx-large">Variety changes the wallpaper on a regular interval</span>'
Comment3=''
License='GPLv3'
Screenshot='http://peterlevi.com/variety/wp-content/uploads/2012/10/01-all-1024x576.jpg'
Url='http://peterlevi.com/variety/'
export Comment
export Name
export Comment3
export License
export Screenshot
export Url

Script A.sh

#!/bin/sh
. $HOME/B.sh
Comment2='Variety changes the desktop wallpaper on a regular basis, using user-specified or automatically downloaded images.'

echo $display_variable_comment3_or_comment2 (depending the case)

sorry my english is bad :/

anubhava · Accepted Answer · 2014-03-13 01:44:51Z

4

In BASH you can do this:

pvar="${s:-defaultVal}"

Then pvar will be set to $s if it exists otherwise it will be set to defaultVal

answered Mar 13, 2014 at 1:44

anubhava

790k67 gold badges603 silver badges671 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

davidva Over a year ago

Excellent! thanks! work great! I only renamed the variable 'Comment3' to 'Comment2' in my script "A.sh"; and also made a new variable called pvar in my script 'A.sh': pvar="${Comment2:-$Comment3}"

hek2mgl · Accepted Answer · 2014-03-13 09:05:19Z

1

It depends on whether you need to check if the variable exists or if it's value is empty. If you need only to test if it is empty you can use this:

if [ -z "$COMMENT3" ] ; then
    COMMENT3="some comment"
fi

... otherwise use @anubhava's answer.

edited Mar 13, 2014 at 9:05

answered Mar 13, 2014 at 1:44

hek2mgl

159k31 gold badges263 silver badges279 bronze badges

2 Comments

ruakh Over a year ago

[ -z $COMMENT3 ] is a bad idea, because the OP's example suggests that $COMMENT3 is likely to contain whitespace and so on. How about [ -z "$COMMENT3" ] instead?

hek2mgl Over a year ago

@ruakh You are right, I missed that

Collectives™ on Stack Overflow

How to check if variable (usually text) Is not defined or empty, if variable is empty uses other variable [duplicate]

2 Answers 2

1 Comment

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

2 Comments

Linked

Related