Question

I have a text file,

a1
a2
b1
b2
c1
c2
...

I want to join by two lines so one can sort it:

a1:a2
b1:b2
c1:c2
...

I’m using bash. the read function will eat the leading space, which is undesired. And I hate to write simple stupid C programs.

Then, I can use tr : "\n" to split the joined file back to two files.

Answer 1

paste -s -d ':\n' file should do it.

For example:

% cat f
a1
a2
b1
b2
% paste -s -d ':\n' f
a1:a2
b1:b2