Issue
I am trying to work out how to make bash (force?) expand variables in a string (which was loaded from a file).
I have a file called "something.txt" with the contents:
hello $FOO world
I then run
export FOO=42
echo $(cat something.txt)
this returns:
hello $FOO world
It didn't expand $FOO even though the variable was set. I can't eval or source the file - as it will try and execute it (it isn't executable as it is - I just want the string with the variables interpolated).
Any ideas?
Solution
I stumbled on what I think is THE answer to this question: the envsubst
command.
envsubst < something.txt
Example: To substitute variables in file source.txt and write it to destination.txt for further processing
envsubst < "source.txt" > "destination.txt"
In case it's not already available in your distro, it's in the
GNU package gettext
.
@Rockallite
- I wrote a little wrapper script to take care of the '$' problem.
(BTW, there is a "feature" of envsubst, explained at https://unix.stackexchange.com/a/294400/7088 for expanding only some of the variables in the input, but I agree that escaping the exceptions is much more convenient.)
Here's my script:
#! /bin/bash
## -*-Shell-Script-*-
CmdName=${0##*/}
Usage="usage: $CmdName runs envsubst, but allows '\$' to keep variables from
being expanded.
With option -sl '\$' keeps the back-slash.
Default is to replace '\$' with '$'
"
if [[ $1 = -h ]] ;then echo -e >&2 "$Usage" ; exit 1 ;fi
if [[ $1 = -sl ]] ;then sl='\' ; shift ;fi
sed 's/\\\$/\${EnVsUbDolR}/g' | EnVsUbDolR=$sl\$ envsubst "$@"
Answered By - LenW Answer Checked By - Senaida (WPSolving Volunteer)