bash Cookbook

Converting Uppercase to Lowercase

Problem

You need to eliminate case distinctions in a stream of text.

Solution

You can translate all uppercase characters (A–Z) to lowercase (a–z) using the tr command and specifying a range of characters, as in:

$ tr 'A-Z' 'a-z' <be.fore >af.ter

There is also special syntax in tr for specifying this sort of range for upper-and lower-case conversions:

$ tr '[:upper:]' '[:lower:]' <be.fore >af.ter

Discussion

Although tr doesn’t support regular expressions, it does support a range of characters. Just make sure that both arguments end up with the same number of characters. If the second argument is shorter, its last character will be repeated to match the length of the first argument. If the first argument is shorter, the second argument will be truncated to match the length of the first.

Here’s a very simplistic encoding of a text message using a simple substitution cypher that offsets each character by 13 places (i.e., ROT13). An interesting characteristic of ROT13 is that the same process is used to both encipher and decipher the text:

$ cat /tmp/joke
Q: Why did the chicken cross the road?
A: To get to the other side.

$ tr 'A-Za-z' 'N-ZA-Mn-za-m' < /tmp/joke
D: Jul qvq gur puvpxra pebff gur ebnq?
N: Gb trg gb gur bgure fvqr.

$ tr 'A-Za-z' 'N-ZA-Mn-za-m' < /tmp/joke | tr 'A-Za-z' 'N-ZA-Mn-za-m'
Q: Why did the chicken cross the road?
A: To get to the other side.

See Also

• man tr

• http://en.wikipedia.org/wiki/Rot13