#001

substitute shell variables in text

Notes

envsubst is is available on most *nix systems and can be installed on MacOS as part of the BSD gettext library.

It’s a useful tool for either completely or selectively substituting environment variables in text that has shell format strings.

Example uses include:

• generating config files from templates with variable substitution

Quick Example

envsubst will attempt to substitute all variable by default:

$ echo 'substituting ${TEST} ${ALT} ${NADA}' | TEST=abc ALT=alt envsubst
substituting abc alt

Or you can give it a constrained list of variables to interpolate:

$ echo 'substituting ${TEST} ${ALT} ${NADA}' | TEST=abc ALT=alt envsubst '$TEST:$ALT'
substituting abc alt ${NADA}

MacOS Installation

Easy to install with homebrew:

$ brew install gettext

But note the formula is keg-only, which means it was not symlinked into /usr/local. So while you could directly reference /usr/local/opt/gettext/bin/envsubst or add that to the path, brew link can be used to add the symlinks:

$ brew link --force gettext
$ which envsubst
/usr/local/bin/envsubst

When done, you can unlink if desired:

$ brew unlink gettext

Credits and References

• envsubst: command not found on Mac OS X 10.8
• man envsubst
• gettext - brew formula doc
• gettext.rb - brew formula source