All examples presented below work for general case where there's arbitrary number of words on the line. Essential idea is the same everywhere - we have to read file line by line, and print words in reverse. AWK facilitates this the best,because it already has all the necessary tools for text processing done programmatically, and is most portable - it can be used witih any awk derivative, and most systems have it. Python also has quite a few good utilities for string processing that allow us to do the job. It's a tool for more modern systems, I'd say. Bash , IMHO, is the least desirable approach, due to portability, potential hazards, and the amount of "trickery" that needs to be done.
AWK
$ awk '{for(i=NF;i>=1;i--) printf "%s ", $i;print ""}' input.txt
Earth Hello
Mars Hello
The way this works is fairly simple: we're looping backwards through each word on the line, printing words separated with space - that's done by printf "%s ",$i function (for printing formated strings) and for-loop. NF variable corresponds to number of fields. The default field separator is assumed to be space. We start by setting throw-away variable i to number of words, and on each iteration decrement the variable. Thus if there's 3 words on line, we print field $3, then $2, and $1. After last pass, variable i becomes 0, the condition i>=1 becomes false, and the loop terminates. To prevent lines being spliced together, we insert a newline using print "". AWK code blocks {} are processed for each line in this case ( if there's a matching condition in front of code block, it depends on the match for the code block to be executed or not).
Python
For those who like alternative solutions, here's python:
$ python -c "import sys;print '\n'.join([ ' '.join(line.split()[::-1]) for line in sys.stdin ])" < input.txt
Earth Hello
Mars Hello
The idea here is slightly different. < operator tells your current shell to redirect input.txt into python's stdin stream, and we read that line by line. Here we use list comprehension to create a list of lines - that's what [ ' '.join(line.split()[::-1]) for line in sys.stdin ] part does. The part ' '.join(line.split()[::-1]) takes a line, splits it into list of words, reverses the list via [::-1] , and then ' '.join() creates a space-separated string out of it. We have as a result a list of larger strings. Finally, '\n'.join() makes even larger string, with each item joined via newline.
In short, this method is basically break and rebuild approach.
BASH
#!/bin/bash
while read line
do
bash -c 'i=$#; while [ $i -gt 0 ];do printf "%s " ${!i}; i=$(($i-1)); done' sh $line
echo
done < input.txt
And a test run:
$ ./reverse_words.sh
Earth Hello
Mars Hello
Bash itself doesn't have strong text processing capabilities. What happens here is that we read the file line by line via
while read line;
do
# some code
done < text.txt
This is a frequent technique and is widely used in shell scripting to read output of a command or a text file line-by-line. Each line is stored into $line variable.
On the inside we have
bash -c 'i=$#; while [ $i -gt 0 ];do printf "%s " ${!i}; i=$(($i-1)); done' sh $line
Here we use bash with -c flag to run a set of commands enclosed into single-quotes. When -c is used, bash will start assigning command-line arguments into variables starting with $0. Because $0 traditionally is used to signify program's name, I use sh dummy variable first.
Unquoted $line will be broken down into individual items due to the behavior known as word-splitting. Word splitting is often undesirable in shell scripting, and you will often hear people say "always quote your variables, like "$foo"." In this case , however, word-splitting is desirable for processing simple text. If your text contains something like $var , this might break this approach. For this , and several other reasons, I'd say python and awk approach are better.
As for the inner code, it's also simple: the unquoted $line is split into words and is passed to inner code for processing. We take number of arguments $#, store it into throw away variable i, and again - print out each item using something known as variable indirection - that's the ${!i} part ( note that this is bashism - it's not available in other shells ). And again, we use printf "%s " to print out each word space separated. Once that's done, echo will insert newline.
Essentially this approach is a mix of both awk and python. We read file line by line, but divide and conquer each line, using several of bash's features to do the job.
Simpler variation can be done with GNU tac command, and again playing with word splitting. tac is used to reverse lines of input stream or file, but in this case we specify -s " " to use space as separator. Thus, var will contain newline-separated list of words in reverse order, but due to $var not being quoted, newline will be substituted with space. Trickery, and again not the most reliable, but works.
#!/bin/bash
while read line
do
var=$(tac -s " " <<< "$line" )
echo $var
done < input.txt
Test runs:
And here's the 3 methods with arbitrary lines of input
$ cat input.txt
Hello Earth end of line
Hello Mars another end of line
abra cadabra magic
$ ./reverse_words.sh
line of end Earth Hello
line of end another Mars Hello
magic cadabra abra
$ python -c "import sys;print '\n'.join([ ' '.join(line.split()[::-1]) for line in sys.stdin ])" < input.txt
line of end Earth Hello
line of end another Mars Hello
magic cadabra abra
$ awk '{for(i=NF;i>=1;i--) printf "%s ", $i;print ""}' input.txt
line of end Earth Hello
line of end another Mars Hello
magic cadabra abra
Extra: perl and ruby
Same idea as with python - we split each line into array of words, reverse the array, and print it out.
$ perl -lane '@r=reverse(@F); print "@r"' input.txt
line of end Earth Hello
line of end another Mars Hello
magic cadabra abra
$ ruby -ne 'puts $_.chomp.split().reverse.join(" ")' < input.txt
line of end Earth Hello
line of end another Mars Hello
magic cadabra abra
