Find empty variables in bash array

Question

I have multiple variables in an array, I want to loop through each of those variables and find if they are empty.

Code:

declare -a CONFIG_PARAM=( $Var_MODE $Var_Path $Var_Stage $Var_line $Var_CVG $Var_Operator )
for PARAMS in "${CONFIG_PARAM[@]}"; do 
   if [ ! $PARAMS ]; then 
      echo -e "$PARAMS is empty"
   fi
done;

But it doesnt seem to catch the empty variables. Can someone point out what is wrong or provide any suggestion/solution? Thanks in advance for any help.

Quotes. "$Var_MODE" "$Var_PATH" etc. And [ ! "$PARAMS" ] — Charles Duffy
– Charles Duffy, Commented Sep 29, 2017 at 16:07
But that's only if you needed the values. If you need the names, then you need to use an indirect expansion. — Charles Duffy
– Charles Duffy, Commented Sep 29, 2017 at 16:07
Thanks I just tried the and got the values but I needed the variable names to be printed! — Gopinath S
– Gopinath S, Commented Sep 29, 2017 at 16:10
Hmm. Arguably this is a duplicate of can I do a for loop over variables in the bash shell — Charles Duffy
– Charles Duffy, Commented Sep 29, 2017 at 16:12

Charles Duffy · Accepted Answer · 2017-09-29 16:39:22Z

3

declare -a config_params=( Var_MODE Var_Path Var_Stage Var_line Var_CVG Var_Operator )
for param in "${config_params[@]}"; do 
   if ! [ -n "${!param}" ]; then 
      echo "$param is empty"
   fi
done

Note:

Always quote expansions. If you don't do this, an empty string doesn't evaluate to anything at all -- so [ ! $empty ] becomes [ ! ] if empty is empty. And [ ! ] is equivalent to [ -n ! ], so it evaluates as true. See http://mywiki.wooledge.org/Quotes, http://mywiki.wooledge.org/Arguments and http://wiki.bash-hackers.org/syntax/words.
Using ${!var} expands the variable whose name is contained in var. See BashFAQ #6.
Avoid all-caps variable names. See relevant POSIX spec at http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html, reserving names with lower-case characters for application use, while indicating that the shell and/or POSIX utilities shall use upper-case variables for their own purposes.

edited Sep 29, 2017 at 16:39

answered Sep 29, 2017 at 16:08

Charles Duffy

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3 Comments

Gopinath S Over a year ago

Thank you so much, this is very helpful. But I have tried the code above, it still prints the values of variables instead of the varible names.

Charles Duffy Over a year ago

@GopinathS, ...uh... no, it doesn't. See this code actually running at ideone.com/4iUpnd

Gopinath S Over a year ago

My bad, I didn't remove '$' for variables in the array, sorry about that. Now I understood the indirect-reference concept from your note. Thanks a lot, appreciate your help.

chepner · Accepted Answer · 2017-09-29 23:23:54Z

This is a case where (IMO) the loop makes your code less readable. There is something to be said for a nice vertical list of calls, one line per variable.

check_empty () {
    [ -z "${!1}" ] && printf '%s is empty\n' "$1"
}

and make a separate call for each variable.

check_empty Var_MODE
check_empty Var_Path
check_empty Var_Stage
check_empty Var_line
check_empty Var_CVG
check_empty Var_Operator

Actually, you don't even need a separate function. The parameter expansion operator :? will display an error message, with the name of the variable, if it is empty.

: ${Var_MODE?empty} || :
: ${Var_Path?empty} || :
: ${Var_Stage?empty} || :
: ${Var_line?empty} || :
: ${Var_CVG?empty} || :
: ${Var_Operator?empty} || :

The trailing || : prevents the script from exiting when the variable is empty.

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