Question

I have a variable with the contents “eth0 eth1 bond0”, is there a way using sed or a similar tool to move anything matching bond.* to the beginning of the line?

Answer 1

Using nothing but Bash:

$ var="eth0 eth1 bond0"
$ [[ $var =~ (.*)\ (bond.*) ]]
$ var="${BASH_REMATCH[2]} ${BASH_REMATCH[1]}"
$ echo "$var"
bond0 eth0 eth1

Edit:

This version handles multiple occurrences of “bond” anywhere within the string:

var="eth0 bond0 eth1 bond1 eth2 bond2"
for word in $var
do
    if [[ $word =~ bond ]]
    then
        begin+="$word "
    else
        end+="$word "
    fi
done
var="$begin$end"
var="${var%* }"    # if you need to strip the trailing space
echo "$var"

Output:

bond0 bond1 bond2 eth0 eth1 eth2

Edit 2:

Here is a sed version that is specific to the string “bond” followed by one or more digits rather than allowing any word that begins with “b”:

$ echo "eth0 bond0 eth1 bond1 eth2 bond2" | 
sed 's/\<bond[[:digit:]]\+/\n&\n/g; s/ //g; :a; s/\(.*\)\n\(bond[[:digit:]]\+\)/\2 \1/;ta; s/\n/ /g'
bond0 bond1 bond2 eth0 eth1 eth2