Useful commands

Environment variables by variable

Imagine we need to access the contents of an environment variable, but its name is stored in another variable.

For example:

DEV_AWS_ACCESS_KEY_ID="1234-dev"
RD_AWS_ACCESS_KEY_ID="abcd-prd"

This could happen for instance while configuring multiple AWS in a CI system.

Let's continue with the example, the CI system provide a variable called "stage", which can be dev or prd; then we want to prepend the content of this variable to get the credentials to the proper account:

CUR_ENV=`echo ${stage} | tr a-z A-Z`
ENV_ACCESS_KEY="${CUR_ENV}_AWS_ACCESS_KEY_ID"

Now the magic comes, if this the shell is based in bash, we could the technique called as "variable indirection", like this:

ACTUAL_KEY=echo ${!ENV_ACCESS_KEY}

However, this will not work on all the shells, a more general solution could be:

eval ACTUAL_KEY=\$$ENV_ACCESS_KEY

However, there might be security implications by using eval

Check if there are git changes in script

In a CI pipeline, you might want to check if there are changes to create an automatic commit, etc.

You can do that by running the following snippet:

git diff-index --quiet HEAD
ANY_CHANGE=$?
[ $ANY_CHANGE -ne 0 ] && echo "Do something with the change"

See strings in binary file

strings login.php.swp