How to force bash to do variable expansion on a string?

Question

I have read a line of bash code from the file, and I want to send it to log. To make it more useful, I'd like to send the variable-expanded version of the line.

I want to expand only shell variables. I don't want the pipes to be interpreted, nor I don't want to spawn any side processes, like when expanding a line with $( rm -r /) .

I know that variable expansion is very deeply woven into the bash. I hope there is a way to perform just expansion, without any side effects, that would come from pipes, external programs and - perhaps - here-documents.

Maybe there is something like eval ?

#!/bin/bash
linenr=42
line=`sed "${linenr}q;d" my-other-script.sh`
shell-expand $line >>mylog.log

I want a way to expand only the shell variables. if x=a , a="example" then I want the following expansions:

echo $x should be echo a .

echo ${a} should be echo example

touch ${!x}.txt should be touch example.txt

if [ (( ${#a} - 6 )) -gt 10 ]; then echo "Too long string" if [ (( ${#a} - 6 )) -gt 10 ]; then echo "Too long string" should be if [ 1 -gt 10 ]; then echo "Too long string" if [ 1 -gt 10 ]; then echo "Too long string"

echo "\\$a and \\$x">/dev/null should be echo "\\$a and \\$x>dev/null"

Answer 1

For those arriving five years later, and, although it is not the best answer to the OP's problem , an answer to the question is as follows. Bash can do indirect parameter expansion:

some_param=a
a=b
echo ${!some_param}
echo $BASH_VERSION
# 5.0.18(1)-release

Answer 2

You are absolutely right, it is very dangerous to use the bash. In fact your command suffers from your problem.

Let us discuss your script in detail:

#!/bin/bash
line=`sed "${42}q;d" my-other-script.sh`
shell-expand $line >>mylog.log

The sed may produce many lines of output, so it is misleading to use the name line .

Then you did not quote $line, this may have obscure effects:

$ x='| grep x'
$ ls -l $x
ls: cannot access |: No such file or directory
ls: cannot access grep: No such file or directory
-rw-rw-r-- 1 foo bar 34493 Nov 19 18:51 x
$

In this case the pipe is not executed, but passed to the program ls. If you have untrusted input, it is very hard to program a robust shell script.

Using eval is evil - I would never suggest using it, especially for such a purpose!

An alternative way would be in perl, iterate over the $ENV array and replace all env keys by the env values. This way you have more control over the things, which may happen.