Using/dev/null
Think of/dev/null as a "black hole". It is very equivalent to a write-only file. All content written to it is lost forever. And trying to read content from it is nothing. However,/dev/null is very useful for both the command line and the script.
Prohibit standard output.
1 Cat $filename >/dev/null
2 # The file content is missing and will not be exported to standard output. |
Prohibit standard error (from Example 12-3).
1 RM $badname 2>/dev/null
2 # This error message [standard error] was thrown into the Pacific Ocean. |
Suppresses standard output and standard error output.
1 Cat $filename 2>/dev/null >/dev/null
2 # if "$filename" does not exist, there will be no error message prompts.
3 # if "$filename" exists, the contents of the file are not printed to standard output.
4 # So therefore, the code above does not output any information at all.
5 #
6 # It is useful when you want to test only the exit code of a command without any output.
7 #
8 #
9 # cat $filename &>/dev/null
# can also be, pointed out by Baris Cicek. |
Deleting contents of a file, but preserving the file itself, with all attendant permissions (from Example 2-1 and Example 2-3):
1 Cat/dev/null >/var/log/messages
2 # : >/var/log/messages has the same effect, but will not produce a new process. (because: is built)
3
4 Cat/dev/null >/var/log/wtmp |
Automatically empties the contents of the log file (especially for those pesky "cookies" sent by a commercial web site):
example 28-1. Hide cookies and no longer used
1 if [f ~/.netscape/cookies] # if present, delete.
2 Then
3 rm-f ~/.netscape/cookies
4 fi
5
6 Ln-s/dev/null ~/.netscape/cookies
7 # Now all cookies will be thrown into the black hole and not stored on the disk. |
Using/dev/zero
Like/dev/null,/dev/zero is also a pseudo file, but it actually produces a continuous stream of NULL (binary 0 streams, not ASCII). The output that is written to it is lost and it is difficult to read a series of NULL from/dev/zero, although this can be done through OD or a hexadecimal editor. The main use of/dev/zero is to create an empty file with a specified length for initialization, just like a temporary swap file.
example 28-2. Create an Exchange temp file with/dev/zero
1 #!/bin/bash
2 # Create an interchange file.
3
The $UID of the 4 root_uid=0 # ROOT user is 0.
5 e_wrong_user=65 # not root?
6
7 File=/swap
8 blocksize=1024
9 minblocks=40
Ten success=0
11
12
13 # This script must be run with root.
If ["$UID"-ne "$ROOT _uid"]
Then
echo; echo "You must is root to run this script." Echo
Exit $E _wrong_user
Fi
19
20
blocks=${1:-$MINBLOCKS} # If the command line is not specified,
22 #+ is set to the default 40 block.
23 # The above sentence equals:
#--------------------------------------------------
# if [-N ' $]
Num # Then
# blocks=$1
# Else
# blocks= $MINBLOCKS
# fi
#--------------------------------------------------
32
33
If ["$blocks"-lt $MINBLOCKS]
KM then
blocks= $MINBLOCKS # At least 40 blocks long.
Panax Notoginseng fi
38
39
echo "Creating swap file of size $blocks blocks (KB)."
The DD If=/dev/zero of= $FILE bs= $BLOCKSIZE count= $blocks # writes zeros to the file.
42
Mkswap $FILE $blocks # to build this file as an interchange file (or swap partition).
Swapon $FILE # Activates the swap file.
45
+ echo "Swap file created and activated."
47
Exit $SUCCESS |
Another application for/dev/zero is to populate a specified size file with 0 for a specific purpose, such as mounting a file system to the loopback device (loopback device) (reference example 13-8) or "safely" deleting a file (reference example 12-55).
example 28-3. Create RAMDisk
1 #!/bin/bash
2 # ramdisk.sh
3
4 # "RAMDisk" is a section of system RAM memory,
5 #+ It can be used as a file system to operate.
6 # Its advantage is that the access speed is very fast (both read and write).
7 # Disadvantage: Volatile, loss of data when the computer restarts or shuts down.
8 #+ will reduce the amount of RAM available to the system.
9 #
10 # So what's the effect of RAMDisk?
11 # Save a larger dataset in RAMDisk, such as a table or dictionary,
12 #+ This can speed up data queries, because it is much faster to look up in memory than to find them on disk.
13
14
E_NON_ROOT_USER=70 # must be run with ROOT.
Rootuser_name=root
17
Mountpt=/mnt/ramdisk
size=2000 # 2K Blocks (can be modified as appropriate)
blocksize=1024 # 1 K (1024 byte) size per block
DEVICE=/DEV/RAM0 # First RAM device
22
Username= ' Id-nu '
If ["$username"!= "$ROOTUSER _name"]
Then
The echo "must is root to run/" ' BaseName $ '/"."
Exit $E _non_root_user
Fi
29
if [!-d ' $MOUNTPT] # test If the mount point already exists,
Then #+ if this script has been running several times, it will not be built again.
mkdir $MOUNTPT #+ Because the front has been established.
-Fi
34
The DD If=/dev/zero of= $DEVICE count= $SIZE bs= $BLOCKSIZE # fills the contents of a RAM device with 0.
36 # Why do you need to do this?
MKE2FS $DEVICE # Create a ext2 file system on a RAM device.
Mount $DEVICE $MOUNTPT # mount the device.
The chmod 777 $MOUNTPT # enables ordinary users to access this ramdisk.
40 # However, only the root can be unloading to load it.
41
The echo "/" $MOUNTPT/"now available to use."
43 # Now RAMDisk even ordinary users can be used to access files.
44
45 # Note that RAMDisk is volatile, so when the computer system restarts or shuts down, the contents of the RAMDisk disappear.
46 #
47 # Copy all you want to save the file to a regular disk directory.
48
49 # After reboot, run this script to set up a RAMDisk again.
50 # only Reload/mnt/ramdisk without other steps will not work correctly.
51
52 # If improved, this script can be placed in/etc/rc.d/rc.local,
53 #+ to automatically set up a ramdisk when the system is started.
54 # This is very suitable for the database server with high speed requirements.
55
0 exit |
Finally, it is worth mentioning that Elf binaries use/dev/zero.
Turn from: http://www.linuxsir.org/main/doc/abs/abs3.7cnhtm/zeros.html
