How do I substitute environment variables when I ouput a file?

Old thread, but this is a recurrent issue. Here is a solution using bash's mapfile:

mateus@mateus:/tmp$ cat input.txt
a = $a
b = $b
mateus@mateus:/tmp$ echo a=$a, b=$b
a=1, b=2
mateus@mateus:/tmp$ function subst() { eval echo -E "$2"; }
mateus@mateus:/tmp$ mapfile -c 1 -C subst < input.txt
a = 1
b = 2

The bash builtin mapfile calls user-defined function subst (see -C/-c options) on each line read from input file input.txt. Since the line contains unescaped text, eval is used tu evaluate it and echo the transformed text to stdout (-E avoid special characters to be interpreted).

IMHO this is a much more elegant than any sed/awk/perl/regex based solution.

Another possible solution is to use shell's own substitution. This looks like a more "portable" way not relaying on mapfile:

mateus@mateus:/tmp$ EOF=EOF_$RANDOM; eval echo "\"$(cat <<$EOF
$(<input.txt)
$EOF
)\""
a = 1
b = 2

Note we use $EOF to minimize conflicting cat's here-document with input.txt content.

Both examples from: https://gist.github.com/4288846

EDIT: Sorry, the first example doesn't handles comments and white space correctly. I'll work on that and post a solution. EDIT2: Fix double $RANDOM eval

As described in the same thread at http://www.issociate.de/board/post/281806/sed_replace_by_enviroment_var_content.html, the "proper" solution would be as follows:

awk '{while(match($0,"[$]{[^}]*}")) {var=substr($0,RSTART+2,RLENGTH -3);gsub("[$]{"var"}",ENVIRON[var])}}1' < input.txt > output.txt

eval approach is fine, but tends to fail on XML files for instance.

Can be done with script (e.g. file name is expand.sh):

while read line; do eval echo \"$line\"; done < $1 > $2

The script may be called like this:

env VAR1=value1 sh expand.sh input_file output_file

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— http://www.issociate.de/board/post/281806/sed_replace_by_enviroment_var_content.html