Inline reordering of substrings via bash - bash

Question

Assume a text file that contains specific lines whose word order should be altered. The words (substrings) are delimited by single whitespaces. The lines to be altered can be identified by their first character (e.g., ">").
# cat test.txt
>3 test This is
foo bar baz
foo bar qux
>2 test This is
foo bar baz
>1 test This is
foo bar qux
What command (probably in awk) would you use to apply the same ordering process across all lines starting with the key character?
# cat test.txt | sought_command
>This is test 3
foo bar baz
foo bar qux
>This is test 2
foo bar baz
>This is test 1
foo bar qux

Answer 1

Here's one way you could do it using awk:
awk 'sub(/^>/, "") { print ">"$3, $4, $2, $1; next } 1' file
sub returns true (1) when it makes a substitution. The 1 at the end is the shortest true condition, to trigger the default action { print }.

Answer 2

According to your example, like this:
awk '$1~"^>" {sub(">","",$1);print ">"$3,$4,$2,$1;next} {print}' test.txt

Answer 3

The tool best suited for simple substitutions on individual lines is sed:
$ sed -E 's/>([^ ]+)( [^ ]+ )(.*)/>\3\2\1/' file
>This is test 3
foo bar baz
foo bar qux
>This is test 2
foo bar baz
>This is test 1
foo bar qux
Awk is the right tool for anything more complicated/interesting. Note that unlike the awk solutions you've received so far the above will continue to work if/when you have more than 4 "words" on a line, e.g.:
$ cat file
>3 test Now is the Winter of our discontent
foo bar baz
foo bar qux
>2 test This is
foo bar baz
>1 test This is
foo bar qux
$ sed -E 's/>([^ ]+)( [^ ]+ )(.*)/>\3\2\1/' file
>Now is the Winter of our discontent test 3
foo bar baz
foo bar qux
>This is test 2
foo bar baz
>This is test 1
foo bar qux
$ awk 'sub(/^>/, "") { print ">"$3, $4, $2, $1; next } 1' file
>Now is test 3
foo bar baz
foo bar qux
>This is test 2
foo bar baz
>This is test 1
foo bar qux