sed: replace strings with variable content

Question

I'm trying to write a script that finds strings in a certain pattern ({?varname}) and replaces them with a corresponding environment variable.

This is what I have so far:

function get_env() {
   var=$1
   echo ${!var}
}

sed -e 's/{?\([a-z]*}\)/'$(get_env '\1')'/g' file.txt

The function works, e.g. get_env LOGNAME --> dotan

Sed returns the value of the function, e.g. if I replace the function's content with echo __$1__ I will get {?logname} --> __LOGNAME__

However put together it doesn't work. It's like the function always returns an empty string.

I'm not sure what causes the problem here. Any ideas?

Answer 1

That command line would run get_env '\1' in a command substitution, and paste its output on the command line of sed. You should probably get an error about that, since \1 is not a valid name for a variable.

You could use Perl and s/\{\?([a-z]+)\}/$ENV{$1}/ to do that; %ENV is a hash that contains the environment variables:

$ export name=you
$ echo "hello {?name}" | perl -pe 's/\{\?([a-z]+)\}/$ENV{$1}/'
hello you

(GNU sed has the e command to run external programs, but it uses runs the whole pattern space, not just the matching part, so it doesn't seem to fit here too well.)

---

Purely in Bash, if you insist (setvar.sh):

#!/bin/bash
while IFS= read -r line ; do
        while [[ $line =~ ^(.*)(\{\?[a-zA-Z0-9_]+\})(.*)$ ]] ; do
                varname=${BASH_REMATCH[2]}
                varname=${varname#"{?"}
                varname=${varname%\}}
                printf -v line "%s%s%s" "${BASH_REMATCH[1]}" "${!varname}" "${BASH_REMATCH[3]}"
        done
        printf "%s\n" "$line" 
done < "${1-/dev/stdin}"

$ echo 'foo {?BASH_VERSION} {?blah} bar' |blah=ABC bash setvar.sh
foo 4.4.12(1)-release ABC bar

Answer 2

This is the solution I came up with

grep -o '{?\S*}' file.txt | sed 's/{?\(.*\)}/\1/' | xargs -I {} sh -c "sed -i -e 's/{?"{}"}/'$"{}"'/' file.txt"

To break it down:

• grep -o '{?\S*}' file.txt - get all the variables to replace
• sed 's/{?\(.*\)}/\1/' - strip the formatting
• xargs -I {} sh -c "sed -i -e 's/{?"{}"}/'$"{}"'/' file.txt" - search and replace for each var seprately