Complement option in tr

Question

I can't understand the complement (-c) option in tr command used along with the replace mode (that is without any other options), for e.g:

echo "a" | tr -c a b

Why does it produce:

abroot@Slack

(ab string with no newline)?

You used a and b as tr arguments, but got e in the output? — techraf, Dec 11, 2016 at 11:22
This answer should help you: superuser.com/questions/1155362/complement-option-in-tr — Alessandro Dotti Contra, Dec 11, 2016 at 13:04
I got ab as output for your code on my machine, as it should be: It replaces everything except a with b, and that includes the newline from echo. I've no idea why you got e instead. — dirkt, Dec 11, 2016 at 19:06
@techraf Sorry I've made a typo - the output is indeed ab. Thx for Your attention. — Mulligan, Dec 11, 2016 at 19:17

techraf · Accepted Answer · 2016-12-12 02:35:16Z

tr command processes all characters, including the non-printing ones.

echo in your example produces an output consisting of two characters:

In your call you ordered tr to replace all characters which complement a (in simple words: other than a) with b, so:

Consider testing it with printf (which does not implicitly add a newline to the end, like echo does)

This produces the same input for tr as echo, so the output is also the same:

printf "a\n" | tr -c a b
ab[~]#

Compare with:

printf "a" | tr -c a b
a[~]#

And:

printf "a\n\n" | tr -c a b
abb[~]#

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