Bash Templating: How to build configuration files from templates with Bash?

Try envsubst

$ cat envsubst-template.txt
Variable FOO is (${FOO}).
Variable BAR is (${BAR}).

$ FOO=myfoo

$ BAR=mybar

$ export FOO BAR

$ cat envsubst-template.txt | envsubst
Variable FOO is (myfoo).
Variable BAR is (mybar).

A heredoc is a builtin way to template a conf file.

STATUS_URI="/hows-it-goin";  MONITOR_IP="10.10.2.15";

cat >/etc/apache2/conf.d/mod_status.conf <<EOF
<Location ${STATUS_URI}>
    SetHandler server-status
    Order deny,allow
    Deny from all
    Allow from ${MONITOR_IP}
</Location>
EOF

Regarding yottsa's answer: envsubst was new to me. Fantastic.

You can use this:

perl -p -i -e 's/\$\{([^}]+)\}/defined $ENV{$1} ? $ENV{$1} : $&/eg' < template.txt

to replace all ${...} strings with corresponding enviroment variables (do not forget to export them before running this script).

For pure bash this should work (assuming that variables do not contain ${...} strings):

#!/bin/bash
while read -r line ; do
    while [[ "$line" =~ (\$\{[a-zA-Z_][a-zA-Z_0-9]*\}) ]] ; do
        LHS=${BASH_REMATCH[1]}
        RHS="$(eval echo "\"$LHS\"")"
        line=${line//$LHS/$RHS}
    done
    echo "$line"
done

. Solution that does not hang if RHS references some variable that references itself:

#!/bin/bash
line="$(cat; echo -n a)"
end_offset=${#line}
while [[ "${line:0:$end_offset}" =~ (.*)(\$\{([a-zA-Z_][a-zA-Z_0-9]*)\})(.*) ]] ; do
    PRE="${BASH_REMATCH[1]}"
    POST="${BASH_REMATCH[4]}${line:$end_offset:${#line}}"
    VARNAME="${BASH_REMATCH[3]}"
    eval 'VARVAL="$'$VARNAME'"'
    line="$PRE$VARVAL$POST"
    end_offset=${#PRE}
done
echo -n "${line:0:-1}"

WARNING: I do not know a way to correctly handle input with NULs in bash or preserve the amount of trailing newlines. Last variant is presented as it is because shells “love” binary input:

1. read will interpret backslashes.
2. read -r will not interpret backslashes, but still will drop the last line if it does not end with a newline.
3. "$(…)" will strip as many trailing newlines as there are present, so I end … with ; echo -n a and use echo -n "${line:0:-1}": this drops the last character (which is a) and preserves as many trailing newlines as there was in the input (including no).