envsubst is a Go package for expanding variables in a string using ${var} syntax.
Includes support for bash string replacement functions. This fork allows you to search Google Secret Manager variables.
Documentation can be found on GoDoc.
envsubst -no-empty < input.tmpl > output.txt
Example with Google Secret Manager
file: input.tmpl
Hello ${USER}
My secret from Google: ${gcp:secretmanager:projects/xxx/secrets/mykey/versions/1}
The flags and their restrictions are:
| Option | Meaning | Type | Default |
|---|---|---|---|
-no-empty |
fail if a variable is set but empty | flag |
false |
| Expression | Meaning |
|---|---|
${var} |
Value of $var |
${#var} |
String length of $var |
${var^} |
Uppercase first character of $var |
${var^^} |
Uppercase all characters in $var |
${var,} |
Lowercase first character of $var |
${var,,} |
Lowercase all characters in $var |
${var:n} |
Offset $var n characters from start |
${var:n:len} |
Offset $var n characters with max length of len |
${var#pattern} |
Strip shortest pattern match from start |
${var##pattern} |
Strip longest pattern match from start |
${var%pattern} |
Strip shortest pattern match from end |
${var%%pattern} |
Strip longest pattern match from end |
${var-default |
If $var is not set, evaluate expression as $default |
${var:-default |
If $var is not set or is empty, evaluate expression as $default |
${var=default |
If $var is not set, evaluate expression as $default |
${var:=default |
If $var is not set or is empty, evaluate expression as $default |
${var/pattern/replacement} |
Replace as few pattern matches as possible with replacement |
${var//pattern/replacement} |
Replace as many pattern matches as possible with replacement |
${var/#pattern/replacement} |
Replace pattern match with replacement from $var start |
${var/%pattern/replacement} |
Replace pattern match with replacement from $var end |
For a deeper reference, see bash-hackers or gnu pattern matching.
${var-default}${var+default}${var:?default}${var:+default}