Evaluating variables in a string

Question

The last line in this script won't work as I expect:

myfile="afile.txt" mycmd='cat $myfile' eval $mycmd echo eval $mycmd 

Here echo prints 'eval cat $myfile'. How can I print 'eval cat afile.txt'?

Answer 1

Let's take things step by step:

When you do this:

mycmd='cat $myfile' 

You prevent the shell from interpolating $myfile. Thus:

$ echo $mycmd cat $myfile 

If you want to allow the interpolation, you can use double quotes:

$ mycmd="echo $myfile"  #Double quotes! $ echo "$mycmd" cat afile.txt 

This, of course, freezes the interpretation of $mycmd when you do an eval.

$ myfile="afile.txt" $ mycmd="echo $myfile" $ echo $mycmd cat afile.txt $ eval $mycmd   #Prints out afile.txt $ myfile=bfile.txt $ eval $mycmd   #Still prints out afile.txt and not bfile.txt 

Compare this to:

$ myfile="afile.txt" $ mycmd='cat $myfile'   #Single quotes hide $myfile from the shell echo $mycmd cat $myfile             #Shell didn't change "$myfile", so it prints as a literal $ eval $mycmd           #Prints out afile.txt $ myfile=bfile.txt $ eval $mycmd           #Now prints out bfile.txt 

What you probably want to do is to evaluate the $mycmd in an echo statement when you echo it:

$ echo $(eval "echo $mycmd") $ cat afile.txt $ myfile=bfile.txt $ echo $(eval "echo $mycmd") cat bfile.txt 

Answer 2

You can write:

eval echo eval $mycmd 

or a bit more robust:

eval echo eval "$mycmd" 

That said, I'd recommend avoiding eval whenever possible; it tends to be very fragile, because there are many complicated steps in Bash command-line processing, and use of eval typically means you will perform those steps more than once. It's hard to keep track of what's really going on.