If you want to use sed, this would work:
~/tmp> str="directory_root /root/config/data/"
~/tmp> echo $str | sed 's|^[^/]*\(/[^/]*/\).*$|\1|'
/root/
Or a single liner (assuming directory_root literal is in the line:)
cat file | sed -e 's|^directory_root \(/[^/]*/\).*$|\1|;tx;d;:x'
Explanation of regex in first example:
s| : using the | as the dilimiter (makes it easier to read in this case)
^ : match beginning of line
[^/]* : match all non / characters (this is greedy so it will stop when it hits the first /.
\( : start recording string 1
/ : match literal /
[^/]* : match all non / charcaters
\) : finish rcording string 1
.* : match everything else to the end of the line
| : delimitter
\1 : replace match with string 1
| : delimitter
In the second example, I appended the ;tx;d;:x which does not echo lines that do not match see here. You can then run this on the entire file, and it will only print the lines it modified.
~/tmp> echo "xx" > tmp.txt
~/tmp> echo "directory_root /root/config/data/" >> tmp.txt
~/tmp> echo "xxxx ttt" >> tmp.txt
~/tmp>
~/tmp> cat tmp.txt | sed -e 's|^directory_root \(/[^/]*/\).*$|\1|;tx;d;:x'
/root/