Expanding known env vers in a file and leaving others - linux

Question

I'm sure there's a really easy way of doing this. I'm trying to take a file which contains some environment variables and expand it so that those which are known are expanded to their values whereas those which are not are left alone.
For example, if my file contained the following:
${I_EXIST}
${I_ALSO_EXIST}
${I_DONT_EXIST}
this would be expanded to:
existValue
alsoExistValue
${I_DONT_EXIST}
I ideally want to do this as simply as possible so I don't want a complex substitution using sed, awk or perl. I'm thinking of something similar to a "Here" file, but apart from the fact that I can't get the syntax right, it also blanks out anything which does not expand. E.g:
cat <<EOF
> ${I_EXIST}
> ${I_ALSO_EXIST}
> ${I_DONT_EXIST}
EOF
existValue
alsoExistValue
(i.e. the last value expands to nothing)
Update
Should really have made clear that I was thinking about potentially more than one substitution per line. One way I did find to do this, if we're not fussed about the variables appearing in the file as ${MYVAR} but maybe MYVAR will do:
m4 $( env | sed 's/\([A-Za-z0-9]*\)=\([\/A-Za-z_0-9:|%*. -#]*\)/-D\1=\2' ) myfile
This uses the M4 preprocessor to substitute all the pairs in your environment. A couple of caviats here:
Sorry about the reg exp stuff. It looks pretty nasty and I'm sure there are nicer ways of expressing this. I found problems if my env vars had spaces in them or any unusual characters that weren't in the set.
Of course this is a blunt substitution tool (which I was trying to avoid) so variable might get substituted when you didn't want it to happen.

Answer 1

#!/bin/bash
while read a;
do
n=$(eval echo $a)
if [[ "$n" == "" ]]
then
echo $a
else
echo $n
fi
done < input
Using this as input
${HOME}
${nonexistent}
Gives
/home/myuser
${nonexistent}

Answer 2

Easy to read? Maybe not. It is short and works though :-)
while read r; do
echo $(eval echo ${r%\}}:-'$r'\})
done < input
Magic used:
http://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion
Edit: Further explanation, I hope it makes some sense.
We use two techniques; from the above docs:
${parameter:−word} If parameter is
unset or null, the expansion of word
is substituted. Otherwise, the value
of parameter is substituted.
And
${parameter%word}
The word is expanded to produce a
pattern just as in filename expansion.
If the pattern matches a trailing
portion of the expanded value of
parameter, then the result of the
expansion is the value of parameter
with the shortest matching pattern
(the ‘%’ case) or the longest matching
pattern (the ‘%%’ case) deleted. ...
We use the fact that the input is just what we can use in the shell, we have ${FOOBAR} but need ${FOOBAR:-'${FOOBAR}'} (Single quotes to avoid expansion).
# echo ${doesntexist:-Hello}
Hello
# doesexist=World
# echo ${doesexist:-Will not be printed}
World
So what we need to inject is :-'${FOOBAR}'
To achieve this we trim the } at the end, add the string, then put another } back afterwards.
# echo $r
${FOOBAR}
# echo ${r%\}}
${FOOBAR
The final \} isn't really necessary, since it's got no beginning in this case, but it's better to be explicit and escape it. (Much like you would escape echo \* even if echo * without any matching files gives you a literal *).
Edit2: This of course doesn't take into account that you wanted to support multiple variables in a single row; or any rows with other stuff in them.

Answer 3

while read name; do echo "$name = " $(eval echo $name); done < file_with_vars.txt
will echo all variables what know.
e.g.
in my file called vv
${PATH}
${HAVENOT}
${LOCALE}
will print
${PATH} = /usr/local/narwhal/bin:/opt/local/bin:/opt/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:~/bin
${HAVENOT} =
${LOCALE} = UTF-8
modify the output format as you wish :)