bash - Can envsubst not do in-place substitution? -

i have config file contains env_variable styled variables.

this file. might contain $examples of text. 

now want variable replaced value saved in actual environment variables. i'm trying this:

export examples=lots envsubst < file.txt > file.txt 

but doesn't work when input file , output file identical. result empty file of size 0.

there must reason this, bash basics i'm not aware of? how achieve want do, ideally without first outputting different file , replacing original file it?

i know can enough sed, when discovered envsubst command thought should perfect use case, i'd use that.

redirects handled shell, not program being executed, , setup before program invoked.

the redirect >output.file has effect of creating output.file if doedn't exist , emptying if does. either way, end empty file, , program's output redirected to.

programs sed capable of "in-place" modification must take filename command-line argument, not redirect.

in case, suggest using temporary file , renaming if went ok.