Ubuntu – How to repeat the content of a file n times

The easiest tool here might be sed. To insert b.txt into a.txt after the 5th line , you could write:

sed '5r b.txt' a.txt

sed reads the file specifiied as argument (a.txt) line by line. All lines get reproduced in the output just as they appeared in the input, unless they get altered by a command.

The 5 is an address (line number) at which the following command shall be executed. The command we use is r, which takes a file name as argument (here b.txt), reads it completely and inserts it into the output after the current line.

As it stands above, this sed command line will only print the output to the terminal, without writing to any files. You can either redirect it to a new file (not any of the input files!) using Bash's output redirection:

sed '5r b.txt' a.txt > c.txt

Or you can directly modify the outer input file a.txt by using sed's -i (for "in-place") switch. If you write it as -i.bak, it will make a backup copy of the original input file with the suffix .bak first:

sed -i '5r b.txt' a.txt

An example:

$ cat a.txt 
January
February
March
April
May
October
November
December

$ cat b.txt 
June
July
August
September

$ sed '5r b.txt' a.txt
January
February
March
April
May
June
July
August
September
October
November
December

The command you need is rev, e.g.:

$ rev <<<pa4080
0804ap
$ echo -e "pa4080_1\npa4080_2" | rev
1_0804ap
2_0804ap

Of course this also works with files:

$ >test echo -e "pa4080_1\npa4080_2"
$ <test rev # see annotation below
1_0804ap
2_0804ap

^{rev filename instead of rev <filename is possible but not preferable, as Stéphane Chazelas explains.}

As part of util-linux rev is installed on every Ubuntu system by default.

This and alternative approaches can be found on HowTo: Reverse a String In Unix / Linux Shell? · nixCraft.

By the way: rev is to echo like tac is to cat, so to reverse the order of lines in a file:

$ >test echo -e "pa4080_1\npa4080_2"
$ <test tac
pa4080_2
pa4080_1
$ <test tac | rev
2_0804ap
1_0804ap