> For the complete documentation index, see [llms.txt](https://linuxstory.gitbook.io/advanced-bash-scripting-guide-in-chinese/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://linuxstory.gitbook.io/advanced-bash-scripting-guide-in-chinese/zheng-wen/part5/29_devproc/29_1_devref1.md). # 29.1 /dev `/dev` 目录包含硬件中可能存在或不存在的*物理设备*的条目。这些条目很恰当地被称为*设备文件*。作为一个例子，包含挂载了文件系统的硬盘分区在 `/dev` 中就有条目，如 [df](https://www.tldp.org/LDP/abs/html/system.html#DFREF) 所示。 ```bash bash$ df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda6 495876 222748 247527 48% / /dev/hda1 50755 3887 44248 9% /boot /dev/hda8 367013 13262 334803 4% /home /dev/hda5 1714416 1123624 503704 70% /usr ``` 其中，`/dev` 目录包含*回环*设备，例如 `/dev/loop0`。回环设备是是一种小花招，它允许像访问块设备一样访问普通文件。这就允许将整个文件系统挂载到单个大文件内。参见[例 17-8](https://www.tldp.org/LDP/abs/html/system.html#CREATEFS) 和[例 17-7](https://www.tldp.org/LDP/abs/html/system.html#ISOMOUNTREF)。 `/dev` 中的一些伪设备有其他的特殊用途，例如 [`/dev/null`](https://www.tldp.org/LDP/abs/html/zeros.html#ZEROSREF)、[`/dev/zero`](https://www.tldp.org/LDP/abs/html/zeros.html#ZEROSREF1)、[`/dev/urandom`](https://www.tldp.org/LDP/abs/html/randomvar.html#URANDOMREF)、`/dev/sda1`（硬盘分区）、`/dev/udp`（*用户数据报文*端口）和 [`/dev/tcp`](#DEVTCP)。 例如： 要手动挂载一个 USB 闪存，应将下面的行添加到 `/etc/fstab`。 ``` /dev/sda1 /mnt/flashdrive auto noauto,user,noatime 0 0 ``` （另请参见[例 A-23](https://www.tldp.org/LDP/abs/html/contributed-scripts.html#USBINST)）。 检查一个磁盘是否在 CD 刻录机中（软链接到 `/dev/hdc`）： ```bash head -1 /dev/hdc # head: cannot open '/dev/hdc' for reading: No medium found # (No disc in the drive.) # head: error reading '/dev/hdc': Input/output error # (There is a disk in the drive, but it can't be read; #+ possibly it's an unrecorded CDR blank.) # Stream of characters and assorted gibberish # (There is a pre-recorded disk in the drive, #+ and this is raw output -- a stream of ASCII and binary data.) # Here we see the wisdom of using 'head' to limit the output #+ to manageable proportions, rather than 'cat' or something similar. # Now, it's just a matter of checking/parsing the output and taking #+ appropriate action. ``` 当对 `/dev/tcp/$host/$port` 伪设备文件执行命令时，Bash 打开一个到关联*套接字*的 TCP 连接。 > *套接字*（*socket*）是与特定 I/O 端口关联的通信节点。（这与连接线的*硬件接口*，或*插座*类似）。它允许同一个机器上、同一个网络的机器之间、不同网络的机器之间以及（当然）位于互联网不同位置的机器之间进行数据传输。 下面的示例假设有一个有效的互联网连接。 从 `nist.gov` 获取时间： ```bash bash$ cat /dev/tcp/www.net.cn/80 bash$ echo -e "GET / HTTP/1.0\n" >&5 bash$ cat <&5 ``` 【感谢 Mark 和 Mihai Maties。】 **例 29-1. 使用 `/dev/tcp` 进行排错** ```bash #!/bin/bash # dev-tcp.sh: /dev/tcp 重定向来验证互联网连接。 # Script by Troy Engel. # Used with permission. TCP_HOST=news-15.net # 一个已知的垃圾邮件友好的 ISP。 TCP_PORT=80 # 端口 80 是 http。 # 尝试连接。（有些类似 “ping”……） echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT} MYEXIT=$? : <摘自 bash 参考： /dev/tcp/host/port 如果 host 是合法的主机名或互联网地址，port 是整数端口号或服务名，Bash 会尝试打开到 对应套接字的 TCP 连接。 EXPLANATION if [ "X$MYEXIT" = "X0" ]; then echo "Connection successful. Exit code: $MYEXIT" else echo "Connection unsuccessful. Exit code: $MYEXIT" fi exit $MYEXIT ``` **例 29-2. 播放音乐** ```bash #!/bin/bash # music.sh # Music without external files # Author: Antonio Macchi # Used in ABS Guide with permission. # /dev/dsp default = 8000 frames per second, 8 bits per frame (1 byte), #+ 1 channel (mono) duration=2000 # If 8000 bytes = 1 second, then 2000 = 1/4 second. volume=$'\xc0' # Max volume = \xff (or \x00). mute=$'\x80' # No volume = \x80 (the middle). function mknote () # $1=Note Hz in bytes (e.g. A = 440Hz :: { #+ 8000 fps / 440 = 16 :: A = 16 bytes per second) for t in `seq 0 $duration` do test $(( $t % $1 )) = 0 && echo -n $volume || echo -n $mute done } e=`mknote 49` g=`mknote 41` a=`mknote 36` b=`mknote 32` c=`mknote 30` cis=`mknote 29` d=`mknote 27` e2=`mknote 24` n=`mknote 32767` # European notation. echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \ $n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" > /dev/dsp # dsp = Digital Signal Processor exit # A "bonny" example of an elegant shell script! ``` ### 注 1. 与以*字符*为单位访问数据的*字符设备*相比，*块设备*以大段或*块*的形式读取和/或写入数据。块设备的例子有硬盘、CD 光盘以及闪存。字符设备的例子有键盘、调制解调器、声卡。 --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://linuxstory.gitbook.io/advanced-bash-scripting-guide-in-chinese/zheng-wen/part5/29_devproc/29_1_devref1.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.