Question

I read a string from a file with shell script variables, and want to substitute the variables with values in a function like

hello.txt:
-------------
Hello $NAME


a.sh 
-------------
function printout
{
  echo ???somehow_parse??? $1
}

NAME=Joe
printout "$(cat hello.txt)"
NAME=Nelly
printout "$(cat hello.txt)"

The example is not the best, but it describes my problem. In other words: can I use shell as a template engine?

I am using ksh.

Answer 1

In general, I would go for a search-and-replace approach using sed/awk such as that shown in Kent’s answer or this answer.

If you want a shell-only approach, then the standard way would be to use eval. However, this poses a security risk. For example:

[me@home]$ cat hello.txt
hello $NAME; uname -a
[me@home]$ NAME="shawn"
[me@home]$ eval echo "`cat hello.txt`"   # DO NOT DO THIS!
hello shawn
Linux SOMEHOST 2.6.9-101.ELsmp #1 SMP Fri May 27 18:57:30 EDT 2011 i686 i686 i386 GNU/Linux

Notice how a command can be injected into the template!

You can however reduce the risk using this approach:

[me@home]$ eval "OUT=\"`cat hello.txt`\""
[me@home]$ echo $OUT
hello shawn; uname -a

Do note that this is still not foolproof as commands can still be injected using $(cmd) or `cmd`.

In short, you should use eval only if you understand the risks and can control/limit access to the template files.

Here’s an example of how this can be applied in your script:

function printout {
    FILENAME=$1
    eval "OUT=\"`cat $FILENAME`\""
    echo $OUT
}

NAME=Joe
printout hello.txt
NAME=Nelly
printout hello.txt