Linux shell 重定向输入和输出
- [1. Standard I/O streams](#1. Standard I/O streams)
- [2. Redirecting to and from the standard file handles (标准文件句柄的重定向)](#2. Redirecting to and from the standard file handles (标准文件句柄的重定向))
-
- [2.1. `command > file`](#2.1.
command > file
) - [2.2. `command >> file`](#2.2.
command >> file
) - [2.3. `command 2> file`](#2.3.
command 2> file
) - [2.4. `command 2>> file`](#2.4.
command 2>> file
) - [2.5. `command < file`](#2.5.
command < file
) - [2.6. `command < infile > outfile`](#2.6.
command < infile > outfile
) - [2.7. `command > file 2>&1`](#2.7.
command > file 2>&1
) - [2.8. `command 2>&1 > file`](#2.8.
command 2>&1 > file
)
- [2.1. `command > file`](#2.1.
- References
In computing, redirection is a form of interprocess communication, and is a function common to most command-line interpreters, including the various Unix shells that can redirect standard streams to user-specified locations.
在计算机领域,重定向是大多数命令行解释器所具有的功能,包括各种可以将标准流重定向用户规定目的地的 Unix shells。
1. Standard I/O streams
In Unix shells derived from the original Bourne shell, the first two actions can be further modified by placing a number (the file descriptor) immediately before the character; this will affect which stream is used for the redirection.
源自 Bourne shell 的许多 Unix shell,可以将一个数字 (文件描述符) 放在重定向符号前,这样可以影响用于重定向的数据流。
The Unix standard I/O streams are:
File Descriptor | Name | Description | 操作符 |
---|---|---|---|
0 | stdin | standard input (标准输入) | < , << |
1 | stdout | standard output (标准输出) | > , >> , 1> , 1>> |
2 | stderr | standard error (标准错误输出) | 2> , 2>> |
**文件描述符 (File Descriptor) 是进程对其所打开文件的索引,形式上是个非负整数。**Linux 会为每个运行的进程都分配这三个文件。
stdin
默认输入源是 Keyboard,stdout
和 stderr
默认输出目的地是 Text Terminal。
默认情况下,command > file
将 stdout
重定向到 file
,command < file
将 stdin
重定向到 file
。
使用 >
或 >>
时,默认为 stdout
(标准输出) 重定向,>
就是 1>
,1
与 >
之间不能有空格。File Descriptor 0
, 1
, 2
与它后面的操作符 >
, >>
, <
, <<
是一个整体。
ls -a -l > yongqiang.txt
等同于 ls -a -l 1> yongqiang.txt
。
(base) yongqiang@yongqiang:~/software$ ls -a -l
total 864112
drwxr-xr-x 5 yongqiang yongqiang 4096 Jun 4 22:26 .
drwxr-xr-x 26 yongqiang yongqiang 4096 Jun 4 22:31 ..
-rwxr-xr-x 1 yongqiang yongqiang 103219356 Jul 14 2023 Miniconda3-latest-Linux-x86_64.sh
-rwxr-xr-x 1 yongqiang yongqiang 47611219 Dec 7 2021 bazel-4.2.1-installer-linux-x86_64.sh
-rwxr-xr-x 1 yongqiang yongqiang 733973045 Feb 18 15:20 pycharm-professional-2023.1.5.tar.gz
(base) yongqiang@yongqiang:~/software$
(base) yongqiang@yongqiang:~/software$ ls -a -l > yongqiang.txt
(base) yongqiang@yongqiang:~/software$
(base) yongqiang@yongqiang:~/software$ cat yongqiang.txt
total 864112
drwxr-xr-x 5 yongqiang yongqiang 4096 Jun 16 15:14 .
drwxr-xr-x 26 yongqiang yongqiang 4096 Jun 4 22:31 ..
-rwxr-xr-x 1 yongqiang yongqiang 103219356 Jul 14 2023 Miniconda3-latest-Linux-x86_64.sh
-rwxr-xr-x 1 yongqiang yongqiang 47611219 Dec 7 2021 bazel-4.2.1-installer-linux-x86_64.sh
-rwxr-xr-x 1 yongqiang yongqiang 733973045 Feb 18 15:20 pycharm-professional-2023.1.5.tar.gz
-rw-r--r-- 1 yongqiang yongqiang 0 Jun 16 15:14 yongqiang.txt
(base) yongqiang@yongqiang:~/software$
在 yongqiang.txt
文件中有 -rw-r--r-- 1 yongqiang yongqiang 0 Jun 16 15:14 yongqiang.txt
行。yongqiang.txt
文件是在 stdout
(标准输出) 重定向之前创建的,需要准备好输出目的地之后,stdout
才会发送到该目的地。
(base) yongqiang@yongqiang:~/software$ ls -a -l
total 864112
drwxr-xr-x 5 yongqiang yongqiang 4096 Jun 16 15:26 .
drwxr-xr-x 26 yongqiang yongqiang 4096 Jun 4 22:31 ..
-rwxr-xr-x 1 yongqiang yongqiang 103219356 Jul 14 2023 Miniconda3-latest-Linux-x86_64.sh
-rwxr-xr-x 1 yongqiang yongqiang 47611219 Dec 7 2021 bazel-4.2.1-installer-linux-x86_64.sh
-rwxr-xr-x 1 yongqiang yongqiang 733973045 Feb 18 15:20 pycharm-professional-2023.1.5.tar.gz
(base) yongqiang@yongqiang:~/software$
(base) yongqiang@yongqiang:~/software$ ls -a -l 1> yongqiang.txt
(base) yongqiang@yongqiang:~/software$
(base) yongqiang@yongqiang:~/software$ cat yongqiang.txt
total 864112
drwxr-xr-x 5 yongqiang yongqiang 4096 Jun 16 15:26 .
drwxr-xr-x 26 yongqiang yongqiang 4096 Jun 4 22:31 ..
-rwxr-xr-x 1 yongqiang yongqiang 103219356 Jul 14 2023 Miniconda3-latest-Linux-x86_64.sh
-rwxr-xr-x 1 yongqiang yongqiang 47611219 Dec 7 2021 bazel-4.2.1-installer-linux-x86_64.sh
-rwxr-xr-x 1 yongqiang yongqiang 733973045 Feb 18 15:20 pycharm-professional-2023.1.5.tar.gz
-rw-r--r-- 1 yongqiang yongqiang 0 Jun 16 15:26 yongqiang.txt
(base) yongqiang@yongqiang:~/software$
<
- 输入重定向,命令默认从键盘获得输入,<
重定向从其它设备获得输入 (文件等)。
>
, 1>
- 输出 stdout
重定向。命令执行输出 stdout
默认打印到屏幕上,>
or 1>
重定向 stdout
到其它输出设备 (文件等)。
<<
- 输入追加重定向
>>
- 输出追加重定向
2. Redirecting to and from the standard file handles (标准文件句柄的重定向)
2.1. command > file
将 command
命令的 stdout
重定向到文件 file
中。如果 file
已经存在,则清空原有文件。
command > file
executes command
, placing the output in file
, as opposed to displaying it at the terminal, which is the usual destination for standard output. This will clobber any existing data in file
.
command > file
命令执行 command
,然后将 stdout
的内容存入 file
。注意任何 file
内的已经存在的内容将被清空,然后写入新内容。
clobber [ˈklɒbə(r)]:vt. 使受到 (严重经济损失),狠揍,猛打,惩罚,彻底战胜 (或击败),狠击,极大地打击 n. 衣服,随身物品
echo "yongqiangcheng" > file.txt
命令清空文件的内容,然后将 "yongqiangcheng"
写入 file.txt
。
(base) yongqiang@yongqiang:~/software$ echo `date` > yongqiang.txt
(base) yongqiang@yongqiang:~/software$ cat yongqiang.txt
Sun Jun 16 20:33:49 CST 2024
(base) yongqiang@yongqiang:~/software$
Note: 使用的是反引号 `, 而不是单引号 '。
2.2. command >> file
将 command
命令的 stdout
重定向到文件 file
中。如果 file
已经存在,则把信息加在原有文件后面。
To append output to the end of the file, rather than clobbering it, the >>
operator is used: command1 >> file
.
command1 >> file
将 stdout
追加到文件末尾,使用 >>
操作符。
echo "yongqiangcheng" > file.txt
命令将 "yongqiangcheng"
追加到 file.txt
的后面。
2.3. command 2> file
For example, command 2> file
executes command
, directing the standard error stream to file.
执行 command
,然后将标准错误 stderr
输出重定向到文件 file
,清空文件中已有内容,然后写入新内容。
2.4. command 2>> file
执行 command
,然后将标准错误 stderr
输出重定向追加到文件 file
末尾。
2.5. command < file
本来需要从键盘获取输入的命令会转移到文件读取内容。
Using command < file
executes command
, with file
as the source of input, as opposed to the keyboard, which is the usual source for standard input.
执行 command
,使用 file
作为用来替代键盘的输入源。
2.6. command < infile > outfile
command < infile > outfile
combines the two capabilities: command
reads from infile
and writes to outfile
.
同时替换输入和输出,执行 command
,从文件 infile
读取内容,然后将 stdout
写入到 outfile
中。
2.7. command > file 2>&1
To write both stderr
and stdout
to file
, the order should be reversed. stdout
would first be redirected to the file
, then stderr
would additionally be redirected to the stdout
handle that has already been changed to point at the file
: command > file 2>&1
.
首先将 stdout
重定向到 file
,然后 stderr
将重定向到已经更改为指向 file
的 stdout
句柄。
In shells derived from csh
(the C shell), the syntax instead appends the &
(ampersand) character to the redirect characters, thus achieving a similar result. The reason for this is to distinguish between a file named 1
and stdout
, i.e. cat file 2>1
vs cat file 2>&1
. In the first case, stderr
is redirected to a file named 1
and in the second, stderr
is redirected to stdout
.
2>&1
将 stderr
重定向到 stdout
。2
和 1
分别是 stderr
和 stdout
的文件描述符。**为了区别普通文件,当将 stderr
和 stdout
作为重定向目的地时,需要在文件描述符前加上 &
。**为了将 stdout
与文件名为 1
的文件区分开来。例如对于 cat file 2>1
和 cat file 2>&1
,前者会将 stderr
重定向至叫做 1
的文件,后者则将其重定向至 stdout
。
A simplified but non-POSIX conforming form of the command, command > file 2>&1
is: command &>file
or command >&file
.
2.8. command 2>&1 > file
It is possible to use 2>&1
before >
but the result is commonly misunderstood. The rule is that any redirection sets the handle to the output stream independently. So 2>&1
sets handle 2
to whatever handle 1
points to, which at that point usually is stdout
. Then >
redirects handle 1
to something else, e.g. a file, but it does not change handle 2
, which still points to stdout
.
可以将 2>&1
放置在 >
前,但是这样并不能达到我们想要的效果。
In the following example, stdout
is written to file
, but stderr
is redirected from stderr
to stdout
, i.e. sent to the screen: command 2>&1 > file
.
References
[1] Yongqiang Cheng, https://yongqiang.blog.csdn.net/
[2] Redirection (computing), https://en.wikipedia.org/wiki/Redirection_(computing)