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    <p>You can use this:</p> <pre><code>perl -p -i -e 's/\$\{([^}]+)\}/defined $ENV{$1} ? $ENV{$1} : $&amp;/eg' &lt; template.txt </code></pre> <p>to replace all <code>${...}</code> strings with corresponding enviroment variables (do not forget to export them before running this script).</p> <p>For pure bash this should work (assuming that variables do not contain ${...} strings):</p> <pre><code>#!/bin/bash while read -r line ; do while [[ "$line" =~ (\$\{[a-zA-Z_][a-zA-Z_0-9]*\}) ]] ; do LHS=${BASH_REMATCH[1]} RHS="$(eval echo "\"$LHS\"")" line=${line//$LHS/$RHS} done echo "$line" done </code></pre> <p>. Solution that does not hang if RHS references some variable that references itself:</p> <pre> #!/bin/bash line="$(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}" </pre> <p><strong>WARNING</strong>: 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:</p> <ol> <li><code>read</code> will interpret backslashes.</li> <li><code>read -r</code> will not interpret backslashes, but still will drop the last line if it does not end with a newline.</li> <li><code>"$(…)"</code> will strip as many trailing newlines as there are present, so I end <code>…</code> with <code>; echo -n a</code> and use <code>echo -n "${line:0:-1}"</code>: this drops the last character (which is <code>a</code>) and preserves as many trailing newlines as there was in the input (including no).</li> </ol>
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    1. POBash Templating: How to build configuration files from templates with Bash?
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    1. POBash Templating: How to build configuration files from templates with Bash?
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    1. COI would change `[^}]` to `[A-Za-Z_][A-Za-z0-9_]` in the bash version to prevent the shell from going beyond strict substitution (e.g. if it tried to process `${some_unused_var-$(rm -rf $HOME)}`).
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      1. USChris Johnsen
    2. CO@Chris Johnsen you are right, updated the answer.
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    3. COIs it safe if environment variable contains some quotes or backslashes or something? I mean perl version.
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      1. USVladislav Rastrusny
 

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