Pros/cons of declare variable in bash

Arct1c_f0x · 02-15-2021, 12:27 PM

Consider the following script:

Code:

#! /usr/bin/bash                                                                              


declare	-i H
declare	-i M

echo -e	"Set alarm hour value"
read H
echo -e	"Set alarm minute value"
read M


echo "$H:$M"

What advantages are there to using the declare command to declare shell variables as opposed to just setting the variables with read. Note that in both cases the observable effect is the same. Is there something that I'm missing? Something that is going on behind the scenes? What advantages are there to either using the declare command or just initializing the variables with the read command?

Thanks

shruggy · 02-15-2021, 12:35 PM

Sort of poor man input validation I guess.

declare -i declares an integer variable. When such variable is assigned a value, arithmetic evaluation is performed.

According to the rules of arithmetic evaluation, if you assign a non-number value to such a variable, and the assigned string can be construed as a variable name, it will be evaluated as if it were the name of another shell variable. If that variable is unset or null, it will evaluate to 0. If assigned value is syntactically wrong for a variable name or for an arithmetic expression, you'll get a syntax error.

teckk · 02-15-2021, 01:34 PM

Code:

declare -i var

var="abc"
echo "$var"
0

var="123"
echo "$var"
123

var=""$var"+abc"
echo "$var"
123

var=""$var"+20"
echo "$var"
143

read -p "Enter number :" num
Enter number :4
echo "$num"
4

read -p "Enter str :" str
Enter str :abc
echo "$str"
abc

shruggy · 02-15-2021, 01:49 PM

Code:

bash -c 'declare -i x;read x <<<"HISTSIZE";echo $x'

or

Code:

bash -c 'declare -i x;read x <<<"SHLVL";echo $x'

MadeInGermany · 02-15-2021, 02:20 PM

I do not use declare -i
It might give unwanted results on wrong input (cast to 0).
I prefer to accept all input then validate like [[ $var =~ ^[0-9]$ ]]

Arct1c_f0x · 02-15-2021, 02:24 PM

Quote:

Originally Posted by shruggy

Code:

bash -c 'declare -i x;read x <<<"HISTSIZE";echo $x'

or

Code:

bash -c 'declare -i x;read x <<<"SHLVL";echo $x'

What does this mean, Shruggy? what is this code saying?

shruggy · 02-15-2021, 02:44 PM

Just two examples of input that could be already defined variables in Bash with numeric values. I mean, try to enter SHLVL in your script instead of hour or minute value.

rnturn · 02-15-2021, 04:45 PM

Quote:

Originally Posted by Arct1c_f0x

Consider the following script:

Code:

#! /usr/bin/bash                                                                              


declare	-i H
declare	-i M

echo -e	"Set alarm hour value"
read H
echo -e	"Set alarm minute value"
read M


echo "$H:$M"

What advantages are there to using the declare command to declare shell variables as opposed to just setting the variables with read.

Good question. I used to declare arrays but then I found that:

Code:

$ A=( $( cat random.txt ) )

worked the same when "A" wasn't declared as it did when it was.

I can't say whether that is true of all possible variations of "define", though. One advantage I can see is that a variables planned usage is right there for the reader to see at the top of the script. (I would never want to sprinkle "define" statements all around in a script. That would lead to madness.)