第 31 章. 零与空

/dev/zero ... /dev/null

用途/dev/null

将/dev/null视为一个 黑洞。它本质上等同于一个只写文件。写入它的任何东西都会消失。尝试从中读取或输出将一无所获。尽管如此，/dev/null在命令行和脚本中都非常有用。

抑制stdout.

cat $filename >/dev/null # Contents of the file will not list to stdout.

抑制stderr的输出（来自 示例 16-3）。

rm $badname 2>/dev/null # So error messages [stderr] deep-sixed.

抑制 两者 的输出stdout和stderr.

cat $filename 2>/dev/null >/dev/null # If "$filename" does not exist, there will be no error message output. # If "$filename" does exist, the contents of the file will not list to stdout. # Therefore, no output at all will result from the above line of code. # # This can be useful in situations where the return code from a command #+ needs to be tested, but no output is desired. # # cat $filename &>/dev/null # also works, as Baris Cicek points out.

删除文件内容，但保留文件本身及其所有相关权限（来自 示例 2-1 和 示例 2-3）

cat /dev/null > /var/log/messages # : > /var/log/messages has same effect, but does not spawn a new process. cat /dev/null > /var/log/wtmp

自动清空日志文件的内容（特别适合处理商业网站发送的那些讨厌的 "cookies"）

示例 31-1. 隐藏 cookie 罐

# Obsolete Netscape browser. # Same principle applies to newer browsers. if [ -f ~/.netscape/cookies ] # Remove, if exists. then rm -f ~/.netscape/cookies fi ln -s /dev/null ~/.netscape/cookies # All cookies now get sent to a black hole, rather than saved to disk.

用途/dev/zero

像/dev/null, /dev/zero一样，是一个伪设备文件，但它实际上产生一个空字符流（二进制 零，而不是 ASCII 类型）。写入/dev/zero的输出会消失，并且实际上很难读取那里发出的空字符，尽管可以使用 od 或十六进制编辑器来完成。的主要用途/dev/zero是创建一个预定长度的已初始化虚拟文件，用作临时交换文件。

示例 31-2. 使用 设置交换文件/dev/zero

#!/bin/bash # Creating a swap file. # A swap file provides a temporary storage cache #+ which helps speed up certain filesystem operations. ROOT_UID=0 # Root has $UID 0. E_WRONG_USER=85 # Not root? FILE=/swap BLOCKSIZE=1024 MINBLOCKS=40 SUCCESS=0 # This script must be run as root. if [ "$UID" -ne "$ROOT_UID" ] then echo; echo "You must be root to run this script."; echo exit $E_WRONG_USER fi blocks=${1:-$MINBLOCKS} # Set to default of 40 blocks, #+ if nothing specified on command-line. # This is the equivalent of the command block below. # -------------------------------------------------- # if [ -n "$1" ] # then # blocks=$1 # else # blocks=$MINBLOCKS # fi # -------------------------------------------------- if [ "$blocks" -lt $MINBLOCKS ] then blocks=$MINBLOCKS # Must be at least 40 blocks long. fi ###################################################################### echo "Creating swap file of size $blocks blocks (KB)." dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks # Zero out file. mkswap $FILE $blocks # Designate it a swap file. swapon $FILE # Activate swap file. retcode=$? # Everything worked? # Note that if one or more of these commands fails, #+ then it could cause nasty problems. ###################################################################### # Exercise: # Rewrite the above block of code so that if it does not execute #+ successfully, then: # 1) an error message is echoed to stderr, # 2) all temporary files are cleaned up, and # 3) the script exits in an orderly fashion with an #+ appropriate error code. echo "Swap file created and activated." exit $retcode

的另一个应用/dev/zero是将指定大小的文件 "清零" 以用于特殊目的，例如在 loopback 设备 上挂载文件系统（参见 示例 17-8）或 "安全地" 删除文件（参见 示例 16-61）。

示例 31-3. 创建 ramdisk

#!/bin/bash # ramdisk.sh # A "ramdisk" is a segment of system RAM memory #+ which acts as if it were a filesystem. # Its advantage is very fast access (read/write time). # Disadvantages: volatility, loss of data on reboot or powerdown, #+ less RAM available to system. # # Of what use is a ramdisk? # Keeping a large dataset, such as a table or dictionary on ramdisk, #+ speeds up data lookup, since memory access is much faster than disk access. E_NON_ROOT_USER=70 # Must run as root. ROOTUSER_NAME=root MOUNTPT=/mnt/ramdisk # Create with mkdir /mnt/ramdisk. SIZE=2000 # 2K blocks (change as appropriate) BLOCKSIZE=1024 # 1K (1024 byte) block size DEVICE=/dev/ram0 # First ram device username=`id -nu` if [ "$username" != "$ROOTUSER_NAME" ] then echo "Must be root to run \"`basename $0`\"." exit $E_NON_ROOT_USER fi if [ ! -d "$MOUNTPT" ] # Test whether mount point already there, then #+ so no error if this script is run mkdir $MOUNTPT #+ multiple times. fi ############################################################################## dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE # Zero out RAM device. # Why is this necessary? mke2fs $DEVICE # Create an ext2 filesystem on it. mount $DEVICE $MOUNTPT # Mount it. chmod 777 $MOUNTPT # Enables ordinary user to access ramdisk. # However, must be root to unmount it. ############################################################################## # Need to test whether above commands succeed. Could cause problems otherwise. # Exercise: modify this script to make it safer. echo "\"$MOUNTPT\" now available for use." # The ramdisk is now accessible for storing files, even by an ordinary user. # Caution, the ramdisk is volatile, and its contents will disappear #+ on reboot or power loss. # Copy anything you want saved to a regular directory. # After reboot, run this script to again set up ramdisk. # Remounting /mnt/ramdisk without the other steps will not work. # Suitably modified, this script can by invoked in /etc/rc.d/rc.local, #+ to set up ramdisk automatically at bootup. # That may be appropriate on, for example, a database server. exit 0

除了以上所有内容之外，/dev/zero是 ELF (Executable and Linking Format) UNIX/Linux 二进制文件所需要的。