Replacement of environment variables or properties in Bash

Earlier I wrote about the automatic installation of Fusion Middleware components using response files. A thing that lacked in my scripts was that although I had a FMW_HOME variable set in my enviroment shell script, the response files had the location hard coded in them. At the time I hadn't had the chance to figure out how to do property/variable replacement in shell. I do know how to do it with ANT. But I figured that installing ANT for only this was a bit too much, since with the installation of FMW you already get ANT as a module.

For an upgrade of my scripts to FMW 12.2.1.1, I did a Google-search on it and found: http://stackoverflow.com/questions/415677/how-to-replace-placeholders-in-a-text-file. The top 2 suggestions were:

1. sed -e "s/\${i}/1/" -e "s/\${word}/dog/" template.txt
2. i=32 word=foo envsubst < template.txt

Although the first option was favoured by many and considered the answer on the querstion, I personally favour the second. It turns out that sed does not accept references to the environment variables as a replacement. And that makes the replacements hardcoded again. The second does accept environment variable references. Actually, if the variable-reference in the template file  is already present in the environment, no actual replacement assignment have to be provided.

So let's say my response file template looks like:

[ENGINE]

#DO NOT CHANGE THIS.
Response File Version=1.0.0.0.0

[GENERIC]

#Set this to true if you wish to skip software updates
DECLINE_AUTO_UPDATES=true

#
MOS_USERNAME=

#
MOS_PASSWORD=<SECURE VALUE>

#If the Software updates are already downloaded and available on your local system, then specify the path to the directory where these patches are available and set SPECIFY_DOWNLOAD_LOCATION to true
AUTO_UPDATES_LOCATION=

#
SOFTWARE_UPDATES_PROXY_SERVER=

#
SOFTWARE_UPDATES_PROXY_PORT=

#
SOFTWARE_UPDATES_PROXY_USER=

#
SOFTWARE_UPDATES_PROXY_PASSWORD=<SECURE VALUE>

#The oracle home location. This can be an existing Oracle Home or a new Oracle Home
ORACLE_HOME=${FMW_HOME}

#Set this variable value to the Installation Type selected. e.g. Fusion Middleware Infrastructure, Fusion Middleware Infrastructure With Examples.
INSTALL_TYPE=Fusion Middleware Infrastructure

#Provide the My Oracle Support Username. If you wish to ignore Oracle Configuration Manager configuration provide empty string for user name.
MYORACLESUPPORT_USERNAME=

#Provide the My Oracle Support Password
MYORACLESUPPORT_PASSWORD=<SECURE VALUE>

#Set this to true if you wish to decline the security updates. Setting this to true and providing empty string for My Oracle Support username will ignore the Oracle Configuration Manager configuration
DECLINE_SECURITY_UPDATES=true

#Set this to true if My Oracle Support Password is specified
SECURITY_UPDATES_VIA_MYORACLESUPPORT=false

#Provide the Proxy Host
PROXY_HOST=

#Provide the Proxy Port
PROXY_PORT=

#Provide the Proxy Username
PROXY_USER=

#Provide the Proxy Password
PROXY_PWD=<SECURE VALUE>

#Type String (URL format) Indicates the OCM Repeater URL which should be of the format [scheme[Http/Https]]://[repeater host]:[repeater port]
COLLECTOR_SUPPORTHUB_URL=

Saved as 'fmw_12.2.1.1.0_infrastructure.rsp.tpl'; note the reference ORACLE_HOME=${FMW_HOME}. And I have set FMW_HOME with an fmw12c_env.sh script, as described in former posts. Then I only have to do:

envsubst < fmw_12.2.1.1.0_infrastructure.rsp.tpl >>fmw_12.2.1.1.0_infrastructure.rsp

To have the file copied to fmw_12.2.1.1.0_infrastructure.rsp with a replaced FMW_HOME variable:

...
#
SOFTWARE_UPDATES_PROXY_PASSWORD=<SECURE VALUE>

#The oracle home location. This can be an existing Oracle Home or a new Oracle Home
ORACLE_HOME=/u01/app/oracle/FMW12211

#Set this variable value to the Installation Type selected. e.g. Fusion Middleware Infrastructure, Fusion Middleware Infrastructure With Examples.
INSTALL_TYPE=Fusion Middleware Infrastructure
...

Couldn't be more simple, I'd say. Nice thing is that this enables me to do more directives. So, learned something again, from a question dated 7,5 years ago...