如何在bash shell中处理变量赋值？

Question

I am confused about how variable assignment to be processed in shell.

For example:

  var=demo
  foo=$var

the $var will be expanded and the $foo will be "demo".

but if write as:

   count=0
   a_${count}=filename

the bash reports: a_0=filename: command not found

From error message, we know a_${count} has expanded to a_0, so why variable assignment can't work? I found also if write in another way:

   count=0
   filename=a_${count}

everything will be ok, what difference between these??

Answer 1

You cannot have an expression on left hand side of variable assignment.

You can use declare instead:

declare a_${count}=filename

Then to verify:

echo "$a_0"
filename

Answer 2

The problem is that variable assignment happens before the expansion, if it is possible. If not, a_$count=filename is expanded and understood as a command to run. You can make it into a command to delay the assignment by using declare :

declare a_$count=filename