Bash interpreting a variable assignment as a command

Question

I've been trying to do something for a couple of days, and I'm stumped; I keep running into the same problem, no matter how I approach this. I have a text file with 2 columns in it; the first is the variable name, the second is the command to be run, with the output being assigned to the variable in the first column. I use read to assign both columns to their own variables, then put the full expression into a new variable and execute it. No matter how I do it, I always get the expression as a command name and the error "command not found."

That's all a little convoluted, so let me show you. The script is:

while read varName varCmd
do
        echo varName is $varName
        echo varCmd is $varCmd
        declare cmd=$varName=$varCmd
        echo Command is $cmd
        "$cmd"
        echo 1st Value is $varFoo
        echo 2nd Value is $varBar

done < testvars.txt

And the text file is:

varFoo  echo foo
varBar  echo bar

Everything works except the assignment execution itself. Here's what I get:

varName is varFoo
varCmd is echo foo
Command is varFoo=echo foo
./testvars.sh: line 8: varFoo=echo foo: command not found
1st Value is
2nd Value is
varName is varBar
varCmd is echo bar
Command is varBar=echo bar
./testvars.sh: line 8: varBar=echo bar: command not found
1st Value is
2nd Value is

It looks like Bash is interpreting the whole thing as one command name (string) and not interpreting the = as an operator.

What can I do to get Bash to correctly interpret the assignment expression correctly?

Answer 1

This is similar to how you can't do this:

name=foo
"$name"=123

or this:

cmd="ls -l | grep ^d"
$cmd

Assignment words are recognized as such before variables are expanded, similarly to how other operators are parsed before variable expansion (and not recognized after that). That's also what other shells do, and what the standard says, so in a lot of senses the "correct" behaviour.

If you want to refer to a variable named in another, there's namerefs (from Bash 4.3), and also the declare command itself can do that. These would assign 123 to a variable called foo:

declare -n p=foo
p=123

or

varname=foo
declare "$varname=123"

However, it often makes more sense to use associative arrays:

key=foo
value=123
declare -A aa
aa["$key"]="$value"

and in case you're storing commands, see

• How can we run a command stored in a variable?
• BashFAQ 050, or "I'm trying to put a command in a variable, but the complex cases always fail!"

Answer 2

Yes use eval, but you must cause the command string to be executed.

eval $varName=$($varCmd)

Or you could ...

declare cmd="$varName=$($varCmd)"
eval $cmd

Answer 3

Use eval, described in man bash.

 eval $varBar