Test if variable is set

[Wim Korten]

Hi all,

What is the best way to test if a variable has been set? (ksh)

Like:
if (variable set?)
do something
fi

Thx

[Nicolas Dumeige]

Hello,

You could test if the variable has any lenght with test :

[ -z "$var" ] && echo "length of string var is zero"

Cheers

Nicolas

[Zeev Schultz]

Doing 'echo $parameter' and 'echo $?'

like here:

[ring]//usr/include/sys:echo $PWD
/usr/include/sys
[ring]//usr/include/sys:echo $?
0
[ring]//usr/include/sys:echo $rest
sh: rest: Parameter not set.
[ring]//usr/include/sys:echo $?
1
[ring]//usr/include/sys:

exit status 1 returns if command exit status wasn't ok (ie parameter doesn't exist).

[Jason Morgan_4]

Try

echo ${variable}

or

if [ ${variable) = ""
then
echo "null"
else
{do something)
fi

[Jean-Luc Oudart]

You can check the varaible length this is close. try this script :
#!/bin/ksh

A1="abcd"
unset A2
A3=""

if [[ ! -z $A1 ]]; then
echo "A1 is set"
else
echo "A1 is not set"
fi

if [[ ! -z $A2 ]]; then
echo "A2 is set"
else
echo "A2 is not set"
fi

if [[ ! -z $A3 ]]; then
echo "A3 is set"
else
echo "A3 is not set"
fi

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A1 is set
A2 is not set
A3 is not set

In fact A3 is set (and empty).

Regards,
Jean-Luc

[KapilRaj]

if [ $VAR ]
then
execute
else
echo .....
fi

Kaps

[Wim Korten]

If I understand all your replies correctly, there is no command to check on the existance op a variable, like you can do test -f for existance of a file.

In my case I want to check if the LANG variable is set. Is there a difference in behavior between:
1) no LANG variable set
2) LANG variable set to nothing (empty string)

THX

[G. Vrijhoeven]

Hi Wim,

env command will list vars. so
TEST=`env | grep -q LANG`
if [ $? -ne 0 ]
then
echo "LANG is not set"
else
echo "LANG is $LANG"
fi

HTH,

Gideon

[Wim Korten]

Hi Gideon,

I was looking at env also, but what if env returns:
...
X=LANGUAGE
NLS_LANG=DUTCH_THE NETHERLANDS.WE8ISO8859P15
LANG=Dutch
...
How can if find only the LANG variable?

[G. Vrijhoeven]

Hi,

env | grep ^LANG the ^states the line must start with LANG


HTH,

Gideon

[Nicolas Dumeige]

Wim,

The shell deal with memory allocation on its own. Therefore, the declaration of a shell variable or the test of its existence can just take the form a value lookup. If you ask the value of a non-existent variable, the shell will tel you it's value is NULL (nothing) but it won't give you any error.

Cheers

Nicolas

[Bill Hassell · Accepted solution]

This is a common question, especially if you use set -u (abort if an undefined variabnle is accessed). Here's how I do it:

set -u

# Set any variables that might not be defined with a default
# value. That way, we can leave set -u and still check unset values

UNSET=IamNOTset
DT=${DT:-$UNSET}
VUE=${VUE:-$UNSET}

# now check if the variable was ever set:

if [ "$VUE" = $UNSET -o "$DT" = $UNSET ]
then
... setup for non-Xwindow stuff
else
... setup for Xwindows
fi

# In case this is a 'sourced' script,
# return the unset variables backup to
# their original state:

[ $DT = $UNSET ] && unset DT
[ $VUE = $UNSET ] && unset VUE

---
The key to handling an unset variable is to use the shell contstruct ${VAR_NAME:-something} which ensures that the variable is defined with a unique value (IamNOTset) so it can be tested later.

[Jean-Luc Oudart]

Othwerwise you can force the value if not set (or nul value) :
${A1:="zxy"}
will initialise A1 if not set (or is null) with "zxy"

Regards,
Jean-Luc

[Mark Grant]

One nasty little construct I've seen is as follows.

[ "A$VAR" = "A" ] && echo "$VAR is not set"

[Jean-Luc Oudart]

Mark,

I think if VAR is set but empty this would return "not set"

Regards,
Jean-Luc

[Mark Grant]

Thanks for that one Jean-Luc, you're right. I obviously got a bit too excited at suddenly and unexpectedly finding some time to ITRC :)

[Wim Korten]

I'll go for:

if ( env|grep -q ^LANG= ); then
unset LANG
fi

THX All for your replies!