tac
tac is used to concatenate files in reverse, line by line.
$ tac numberedlines.txt
Line number five
Line number four
Line number three
Line number two
Line number one
Useful Options / Examples
-s, --separator
$ cat numberedlines.txt
START1
Line number one
Line number two
END2
START3
Line number three
Line number four
END4
START5
Line number five
Line number six
END6
START7
Line number seven
Line number eight
END8
START8
Line number nine
Line number ten
END9
$ tac -s "START" numberedlines.txt
8
Line number nine
Line number ten
END9
7
Line number seven
Line number eight
END8
START5
Line number five
Line number six
END6
START3
Line number three
Line number four
END4
START1
Line number one
Line number two
END2
START
START$
Break it down
The -s, --separator flag uses the string to demarcate a “line”. So it detects the last instance and prints everything following it until the end of the line. Then it detects the second to last instance and prints lines after it until the next instance of the string including the line separating string, and so on. Output has weird behavior at the edges of the file, use -b, --before.
-b, --before
Using same file as before
$ tac -b -s "START" numberedlines.txt
START8
Line number nine
Line number ten
END9
START7
Line number seven
Line number eight
END8
START5
Line number five
Line number six
END6
START3
Line number three
Line number four
END4
START1
Line number one
Line number two
END2
Break it down
The -b, --before flag attaches the line separating string before each line of output instead of after.