$ envsub [options]
The envsub program substitutes the values of environment variables.
Unlike envsubst, which only supports ${NAME} style substitution, the envsub program also supports ${NAME-default} and ${NAME:-default} style substitution.
The program pipes stdin to stdout one line at a time applying the required substitutions.
-h, --help:: Displays the help details of the command and exit.
-p, --prefix:: Allows configuring an alternative prefix for substitution patterns if the default of ${ would cause conflicts.
-s, --suffix:: Allows configuring an alternative suffix for substitution patterns if the default of } would cause conflicts.
NOTE: If your suffix starts with : or - you will not have a good time as this will conflict with the default value separator that is internal to the pattern.
-g, --greedy-defaults:: Enables greedy replacement of unmatched default values. With this option if you have ${FOO} and there is no corresponding variable FOO then that will remain untouched but ${FOO:-BAR} or ${FOO-BAR} will be replaced as BAR whereas without this option they would remain untouched.
-v, --var:: Only replace the named variable. Can be specified multiple times if you want to replace multiple variables. If not specified then all variables from the environment can be substituted.
-V, --version:: Display the program version and exit.
TIP: When running envsub without the -v argument then only patterns that have environment variables will be searched for, so the ${NAME-default} form will never be replaced if NAME is not defined.
Basic usage:
$ (echo 'FOO=${FOO} or ${FOO-unset} or ${FOO:-empty}') | (unset FOO && envsub)
FOO=${FOO} or ${FOO-unset} or ${FOO:-empty}
$ (echo 'FOO=${FOO} or ${FOO-unset} or ${FOO:-empty}') | (unset FOO && envsub -v FOO)
FOO=${FOO} or unset or empty
$ (echo 'FOO=${FOO} or ${FOO-unset} or ${FOO:-empty}') | (FOO= envsub)
FOO= or or empty
$ (echo 'FOO=${FOO} or ${FOO-unset} or ${FOO:-empty}') | (FOO= envsub -v BAR)
FOO=${FOO} or ${FOO-unset} or ${FOO:-empty}
$ (echo 'FOO=${FOO} or ${FOO-unset} or ${FOO:-empty}') | (FOO=123 envsub)
FOO=123 or 123 or 123
Custom prefixes and suffixes:
$ (echo 'FOO=%FOO:-empty%') | (FOO= envsub -p '%' -s '%')
FOO=empty