Tr

tr stands for translate characters and that's what it does.

tr and changing /dev/zero
If you want a stream of characters like /dev/zero, tr can help you out. Pretend this:

francisco$ dd if=/dev/zero count=1 | tr '\000' '\377'| hexdump -C 1+0 records in 1+0 records out 512 bytes transferred in 0.000 secs (579841 bytes/sec) 00000000 ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff  |ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ| * 00000200

As one can see the stream from 0x0 has changed to 0xff.

tr and sectioning numerics
Pretend you want to see all numbers in a document and count them then here is a tr syntax:

francisco$ tr -c -d '0-9' < testprog.c | hexdump -C 00000000 38 31 39 32 30 30 30 31  38 31 39 32 38 31 39 32  |8192000181928192| 00000010  38 31 39 32 31 30 30 30  30                       |819210000| 00000019 francisco$

Only numbers are outputted from a .c program file.

tr and removing newlines
francisco$ more file line1 line2 line3 francisco$ tr -d '\n' < file | hexdump -C 00000000 6c 69 6e 65 31 6c 69 6e  65 32 6c 69 6e 65 33     |line1line2line3| 0000000f