Reverse order of dot-delimited elements in a string

• Using combination of tr+tac+paste

  $ tr '.' $'\n' <<< 'arg1.arg2.arg3.arg4.arg5' | tac | paste -s -d '.'
  arg5.arg4.arg3.arg2.arg1
  

• If you still prefer bash, you could do in this way,

  IFS=. read -ra line <<< "arg1.arg2.arg3.arg4."
  let x=${#line[@]}-1; 
  while [ "$x" -ge 0 ]; do 
        echo -n "${line[$x]}."; 
        let x--; 
  done
  

• Using perl,

  $ echo 'arg1.arg2.arg3.arg4.arg5' | perl -lne 'print join ".", reverse split/\./;'
  arg5.arg4.arg3.arg2.arg1
  

If you do not mind a tiny bit of python:

".".join(s.split(".")[::-1])

Example:

$ python3 -c 's="arg1.arg2.arg3.arg4.arg5"; print(".".join(s.split(".")[::-1]))'
arg5.arg4.arg3.arg2.arg1

• s.split(".") generates a list containing . separated substrings, [::-1] reverses the list and ".".join() joins the components of the reversed list with ..

You can do it with a single sed invocation:

sed -E 'H;g;:t;s/(.*)(\n)(.*)(\.)(.*)/\1\4\5\2\3/;tt;s/\.(.*)\n/\1./'

this uses the hold buffer to get a leading newline in the pattern space and uses it to do permutations until the newline is no longer followed by any dot at which point it removes the leading dot from the pattern space and replaces the newline with a dot.
With BRE and a slightly different regex:

sed 'H
g
:t
s/\(.*\)\(\n\)\(.*\)\(\.\)\(.*\)/\1\4\5\2\3/
tt
s/\(\.\)\(.*\)\n/\2\1/'

If the input consists of more than one line and you want to reverse the order on each line:

sed -E 'G;:t;s/(.*)(\.)(.*)(\n)(.*)/\1\4\5\2\3/;tt;s/(.*)\n(\.)(.*)/\3\2\1/'