Unix swap fields separated by :

Unix homework 1

Requirement: Using sed(Stream editor) write a command which swaps the first and third field in a given file. The fields in the file are separated by colon (:)

Solution: sed 's/^\([^:]*:\)\([^:]*:\)\([^:]*\)/\3:\2\1/;s/:$//' -i your_filename

Explanations:

'^' start at the beginning of the line

\( \) a grouping 

[^:] any character except ':' 

* zero or more times 

: the character ':'

;s/:$// in here ; is used to separate commands, s/:$// is used to remove last :
-i is used to directly edit the file, if you remove -i then the result will be displayed on the screen instead of editing the file

Example of file content:

field1:field2:field3

somefield1:somefield2:somefield3

After running the command:

field3:field2:field1

somefield3:somefield2:somefield1

References:
http://unix.stackexchange.com/questions/35515/swap-two-columns-in-a-csv-using-sed
http://www.linuxquestions.org/questions/programming-9/remove-last-character-from-file-string-345339/