When trying to redirect to /dev/null and /dev/zero, the output it is discarded. It seems both /dev/null and /dev/zero accept and discard all input. So, what is the difference between /dev/null and /dev/zero?
1 Answer
Yes, both accept and discard all input, but their output is not the same:
/dev/nullproduces no output./dev/zeroproduces a continuous stream of NULL (zero value) bytes.
You can see the difference by executing cat /dev/null and cat /dev/zero.
Try
cat /dev/null > fileand you will find an emptyfile.Now try
cat /dev/zero > file, while watching the size of the file (watch -n 1 du -h file) continuously increase. This is because reading from/dev/zerogives an endless stream of\0(null) characters.
Use dd to visualize the difference more appropriately:
$ dd if=/dev/null of=file count=10
0+0 records in
0+0 records out
0 bytes (0 B) copied, 0.000276193 s, 0.0 kB/s
$ dd if=/dev/zero of=file count=10
10+0 records in
10+0 records out
5120 bytes (5.1 kB) copied, 0.00090775 s, 5.6 MB/s
/dev/zero is used to create dummy files or swap.
Also visit:
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3Worth noting:
mmapping/dev/zerowithMAP_PRIVATEis the "portable" way to obtain an anonymous memory mapping (in the absence of extensions likeMAP_ANON).– nneonneoJan 10, 2016 at 23:11 -
12Perhaps not obvious to the casual reader is just how
/dev/nullproduces no output: It signals EOF immediately. Jan 11, 2016 at 4:30 -
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5hi @AaronFranke, Oh sure!
:-)Consider output of/dev/zeroas a stream of potential all-ones bytes: we just need to replace each all-zeros byte with an all-ones byte. An all-ones byte is377in octal (asprintf '%o\n' $((2#11111111))tells us);tr '\000' '\377' </dev/zero >/tmp/all-ones.datwill swiftly generate a huge pile of all-ones bytes. Jan 12, 2021 at 6:03
rm -f /dev/zero ; echo -n 111111111111 > /dev/zero*runs away*