[Linux]/dev/null and/dev/zero

Use /Dev/null

Set/Dev/nullIt is regarded as a "black hole". It is very equivalent to a write-only file. All the content written to it will be lost forever. However, if you try to read the content from it, nothing can be read. However,/Dev/nullCommand Line and script are very useful.

DisableStandard output.

1 cat $ FILENAME>/dev/null 2 # file content is lost without being output to standard output.

DisableStandard Error(From example 12-3 ).

1 RM $ badname 2>/dev/null 2 # the error message [standard error] is thrown to the Pacific Ocean.

Disable Standard output and standard error output.

1 cat $ filename 2>/dev/null 2 # If "$ FILENAME" does not exist, no error message is prompted. 3 # If "$ FILENAME" exists, the file content will not be printed to the standard output. 4 # therefore, the above Code will not output any information. 5 # 6 # It is useful when you only want to test the exit code of the command and do not want to have any output. 7 # 8 # 9 # Cat $ filename &>/dev/null 10 # Yes, as specified 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 no new process will be generated (because: built-in) 3 4 CAT/dev/null>/var/log/wtmp

Automatically clear the content of log files (especially suitable for processing these annoying "cookies" sent by commercial web sites "):

Example 28-1. Hide the cookie and stop using it

1 If [-f ~ /. Netscape/cookies] # Delete the cookies if they exist. 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 instead of stored on the disk.

Use /Dev/zero

Image/Dev/nullSame,/Dev/zero is also a pseudo FileBut it actually produces continuous null streams (Binary zero streams instead of ASCII). The output written to it will be lost/Dev/zeroReading a series of null statements is also difficult, although it can also be done through OD or a hexadecimal editor./Dev/zeroIt is mainly used to create an empty file with a specified length for initialization, just like a temporary swap file.

Example 28-2.Use/dev/zero to create a temporary file for swap

1 #! /Bin/bash 2 # create a swap file. 3 4 root_uid = 0 # the root user's $ uid is 0. 5 e_wrong_user = 65 # Not root? 6 7 file =/swap 8 blocksize = 1024 9 minblocks = 40 10 success = 0 11 12 13 # The script must be run as root. 14 if ["$ uid"-ne "$ root_uid"] 15 then 16 echo; echo "you must be root to run this script."; echo 17 exit $ e_wrong_user 18 fi 19 20 21 blocks =$ {1:-$ minblocks} # If the command line is not specified, 22 # + is set to the default 40. 23 # The above sentence is equivalent: 24 #-------------------------------------------------- 25 # If [-n "$1"] 26 # Then 27 # blocks = $1 28 # else 29 # blocks = $ minblocks 30 # fi 31 #-------------------------------------------------- 32 33 34 if ["$ blocks"-lt $ minblocks] 35 then 36 blocks = $ minblocks # a minimum length of 40 blocks is required. 37 fi 38 39 40 echo "creating swap file of size $ blocks (KB )." 41 dd If =/dev/Zero of = $ file BS = $ blocksize COUNT = $ blocks # writes zero to a file. 42 43 mkswap $ File $ blocks # create this file as a swap file (or a swap partition ). 44 Swapon $ file # activate the swap file. 45 46 echo "swap file created and activated ." 47 48 exit $ success

About/Dev/zeroAnother application is to fill a file of the specified size with zero fill for a specific purpose, such as mounting a file system to the loopback device (see example 13-8) or "Safely" delete an object (see example 12-55 ).

Example 28-3. Create ramdisk

1 #! /Bin/bash 2 # ramdisk. Sh 3 4 # "ramdisk" is a part of the system RAM memory, 5 # + It can be operated as a file system. 6 # its advantage is that the access speed is very fast (including reading and writing ). 7 # disadvantages: Volatile. Data is lost when the computer is restarted or shut down. 8 # + will reduce the available system Ram. 9 # 10 # What is the role of ramdisk? 11 # Save a large dataset on ramdisk, such as a table or dictionary, 12 # + this will accelerate data query, because searching in memory is much faster than searching in disk. 13 14 15 e_non_root_user = 70 # It must be run with root. 16 rootuser_name = root 17 18 mountpt =/mnt/ramdisk 19 size = 2000 #2 k blocks (which can be modified as appropriate) 20 blocksize = 1024 # each block has a size of 1 K (1024 bytes) 21 device =/dev/ram0 # first ram device 22 23 username = 'id-nu' 24 if ["$ username "! = "$ Rootuser_name"] 25 then 26 echo "must be root to run \" 'basename $0 '\"." 27 exit $ e_non_root_user 28 fi 29 30 if [! -D "$ mountpt"] # test whether the mount point already exists, 31 then # + If the script has been run several times, the directory will not be created again. 32 mkdir $ mountpt # + because it has been created earlier. 33 fi 34 35 dd If =/dev/Zero of = $ device COUNT = $ size BS = $ blocksize # Fill the content of the ram device with zero. 36 # Why? 37 mke2fs $ device # create an ext2 File System on the RAM device. 38 Mount $ device $ mountpt # mount the device. 39 chmod 777 $ mountpt # enable normal users to access this ramdisk. 40 # However, it can only be uploaded by the root user. 41 42 echo "\" $ mountpt \ "now available for use ." 43 # Now ramdisk can be used to access files even for common users. 44 45 # note that ramdisk is easy to lose, so the content in ramdisk disappears when the computer system is restarted or shut down. 46 # 47 # copy all the files you want to save to a regular disk directory. 48 49 # Run the script to create a ramdisk again after restart. 50 # Only reload/mnt/ramdisk and no other steps will work correctly. 51 52 # If it is improved, this script can be placed in/etc/rc. d/rc. Local, 53 # + to automatically set up a ramdisk when the system starts. 54 # This is suitable for database servers with high speed requirements. 55 56 exit 0

It is worth mentioning that,The ELF binary file uses/dev/zero..