Some shell tips -- 14 -- rev

This blog is part of a shell tips list I find useful to use on every script -- the whole list can be found here.

In the situational Unix/Linux commands family, one would most likely be on top of the list: rev. Indeed, rev has no option and does one and unique thing: it reverse characters. Let's have a look at an example:

$ echo "abcdef" | rev
fedcba
$

It also works with files:

$ rev afile.txt
1 enil elifa
2 enil elifa
$

And with heredocs as well:

$ cat << EOF | rev
heredoc line 1
heredoc line 2
EOF
1 enil codereh
2 enil codereh
$

And the result is even more interesting when using tac instead of cat

$ tac << EOF | rev
heredoc line 1
heredoc line 2
EOF
2 enil codereh
1 enil codereh
$

I would not argue that this is useful on a daily basis though :) You can use rev to sort a list by the last character:

 $ cat << EOF | rev | sort | rev
file1.txt
file2.log
file3.csv
EOF
file2.log
file1.txt
file3.csv
$

You could also use rev to have the last character of a strong in uppercase for example by using an extra variable and variable substitution:

$ VAR=$(echo "abcdef" | rev)
$ echo "${VAR^}" | rev
abcdeF
$

But to be fair we could also do this with sed:

$ echo "abcdef" | sed 's/.$/\U&/'
abcdeF
$

And if you would like to upper case the first and the last character of a string, you would do it as below:

$ echo "abcdef" | sed -e 's/\(^.\)\(.*\)\(.$\)/\U\1\L\2\U\3/'
AbcdeF
$

But this is another story :)

To sum up, you will most likely not use rev on a daily basis but it is still a command worth to know as you will need it one day for sure!

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