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 eval $mycmd prints eval cat $myfile.
How can I print eval cat afile.txt?
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4 Answers
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
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.
Comments
You need to eval the echo, rather than the other way around:
eval echo "eval $mycmd"
or
eval echo eval "$mycmd"
I think the former is preferable, but at least quote the variable expansion.
Comments
If you're using bash, the better way to do it is to use arrays:
myfile="afile.txt"
mycmd=(cat "$myfile")
echo "${mycmd[@]}"
"${mycmd[@]}"
