Complement option in tr

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)?

6

1 Answer

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

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

• a
• newline character (\n)

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

• it left the a character intact
• replaced the newline character with b.

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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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