Replacing environment variables in a properties file - linux

Question

In Linux, say I have the following file (e.g. conf.properties):
HOST_URL=http://$HOSTNAME
STD_CONFIG=http://$HOSTNAME/config
USER_CONFIG=http://$HOSTNAME/config/$unconfigured
I want to create another file with all the environment variables replaced...e.g. say the environment variable $HOSTNAME is 'myhost' and $unconfigured is not set, a script should produce the following output:
HOST_URL=http://myhost
STD_CONFIG=http://myhost/config
USER_CONFIG=http://myhost/config/
I was thinking this could be done in a simple one-liner with some sort of sed/awk magic, but I'm no expert and my searches have been in vein, so appreciate any help.
Edit:
I should mention that the file can really be any format text file, for example xml. I just want to replace anything that looks like an env variable with whatever is currently set in the environment.

Answer 1

This is what envsubst is for.
echo 'Hello $USER'
Hello $USER
echo 'Hello $USER' | envsubst
Hello malvineous
You would probably use it more like this though:
envsubst < input.txt > output.txt
envsubst seems to be part of GNU gettext.

Answer 2

sed 's/$HOSTNAME/myhost/g;s/$unconfigured//g' yourfile.txt > another_file.txt
update:
Based on updates in your question, this won't be a good solution.
update2 :
This is based on an answer to a related question. I've hacked at it (I'm unfamiliar with perl) to remove undefined vars.
perl -p -e 's/\$\{([^}]+)\}/defined $ENV{$1} ? $ENV{$1} : $&/eg; s/\$\{([^}]+)\}//eg' yourfile.txt
Should work for any input text file, however you will need to define vars using the ${...} format which simplifies the string matching.
(rant regarding the evilness of eval moved to a separate post so as not to confuse readers)

Answer 3

"eval is evil"
This is not an answer, but a warning in response to using eval for this task. You really really really don't want to do that.
Exhibit 1: a malicious template file:
HOST_URL=http://$HOSTNAME
STD_CONFIG=http://$HOSTNAME/config
USER_CONFIG=http://$HOSTNAME/config/$unconfigured
&& cat /etc/redhat-release
An unsuspecting user:
[lsc#aphek]$ cat somefile | while read line; do echo $(eval echo `echo $line`); done
HOST_URL=http://xyz
STD_CONFIG=http://xyz/config
USER_CONFIG=http://xyz/config/
Red Hat Enterprise Linux WS release 4 (Nahant Update 9)
Note the last line!
Now, imagine the possibilities....

Answer 4

I'd do it like this:
# Set the $HOSTNAME and other variables
# Now evaluate the properties file as a shell script.
. config.properties
# Write the values
cat >somefile <<EOF
HOST_URL=$HOST_URL
STD_CONFIG=$STD_CONFIG
USER_CONFIG=$USER_CONFIG
EOF
Edit: Or this very nasty thing (I'm sure there's a better way)
for name in HOST_URL STD_CONFIG USER_CONFIG
echo "$name=$(eval echo `echo '$'$name`)" >>somefile
end

Answer 5

Thanks to #DarkDust I came up with this:
cat somefile | while read line; do echo $(eval echo `echo $line`); done > somefile.replaced

Answer 6

I used this oneliner to replace ${VARIABLE} style variables in a file:
TARGET_FILE=/etc/apache2/apache2.conf; for VARNAME in $(grep -P -o -e '\$\{\S+\}' ${TARGET_FILE} | sed -e 's|^\${||g' -e 's|}$||g' | sort -u); do sed -i "s|\${$(echo $VARNAME)}|${!VARNAME}|g" ${TARGET_FILE}; done
I'm pretty sure someone can do this in 1/3rd of the length using awk… feel challenged! ;)

Answer 7

Here is a snippet of Javascript that I like to have around for solving this exact problem:
// A Javascript version of envsubst for our builds
// Purpose: replace all ocurrences of ${VAR} with the equivalent var from the environment from stdin
var readline = require('readline');
var rl = readline.createInterface({
input: process.stdin,
output: process.stdout,
terminal: false
});
const environment = process.env;
rl.on('line', function(line) {
const newLine = line.replace(/\$\{([a-zA-Z0-9_]+)\}/g, function(_match, variable) {
const envVar = environment[variable];
return envVar ? envVar : '';
});
process.stdout.write(`${newLine}\n`);
});
Hopefully this helps somebody else.

Answer 8

Here's a short one-liner that uses python's curly brace formatting to safely do the magic:
contents=\"\"\"`cat $file`\"\"\"; python -c "import os;print $contents.format(**os.environ)"
avoids evil eval
allows outputting curly braces: use {{ instead of {
no need to specify vars explicitly when calling the script
For example, given properties file settings.properties:
# my properties file
someVar = {MY_ENV_VAR}
curlyBraceVar = has {{curly braces}}
Then, do the substitution with:
$ export MY_ENV_VAR="hello"
$ file=settings.properties
$ contents=\"\"\"`cat $file`\"\"\"; python -c "import os;print $contents.format(**os.environ)"
# my properties file
someVar = hello
curlyBraceVar = has {curly braces}
A script is here: https://raw.githubusercontent.com/aneilbaboo/machome/master/bin/substenv

Answer 9

if you have installed nodejs you can run
npx #utft/tt -e FOO=bar /path/to/input /path/to/output
or you can run it programmatically
https://github.com/utftufutukgyftryidytftuv/tt