重定向操作

一、将输出重定向到文件或程序。

注意：重定向操作的文件顺序非常重要

> file 2>&1

将标准输出重定向到file，然后将标准错误作为标准输出重定向到相同位置（file）。

2>&1 > file

与以上相反，将标准错误重定向到标准输出的默认位置（终端窗口，因此没有任何更改），然后仅将标准输出重定向file。

示例

1、使用date命令查看的时间戳保存到/tmp/dateback中，供日后参考。

[[email protected] ~]$ date > /tmp/dateback

[[email protected] ~]$ cat /tmp/dateback
Wed  3 Aug 19:55:43 CST 2022

2、将一个日志文件最后100行复制到另一个文件。

[[email protected] ~]$ sudo tail -n 100 /var/log/messages > /tmp/end-100

[[email protected] ~]$ wc -l /tmp/end-100
100 /tmp/end-100

3、将4个文件连接为1个文件。

[[email protected] log]$ sudo cat lastlog maillog tallylog > /tmp/old-log

4、将主目录的隐藏文件名和常规文件名列出到文件中。

[[email protected] ~]$ ls -a > /tmp/my-file

5、将输出附加到现有文件。

[[email protected] ~]$ echo "new line of information" >> /tmp/many-lines
[[email protected] log]$ sudo diff lastlog tallylog >> /tmp/tracking-mode

6、在终端上查看普通命令输出时，将错误重定向到文件。

[[email protected] ~]$ find /etc -name passwd 2> /tmp/errors
/etc/pam.d/passwd
/etc/passwd

[[email protected] ~]$ cat /tmp/errors
find: ‘/etc/lvm/archive’: Permission denied
find: ‘/etc/lvm/backup’: Permission denied
find: ‘/etc/lvm/cache’: Permission denied
find: ‘/etc/pki/rsyslog’: Permission denied
find: ‘/etc/pki/pesign’: Permission denied
find: ‘/etc/ssh/sshd_config.d’: Permission denied
find: ‘/etc/grub.d’: Permission denied
find: ‘/etc/sssd’: Permission denied
find: ‘/etc/polkit-1/rules.d’: Permission denied
find: ‘/etc/polkit-1/localauthority’: Permission denied
find: ‘/etc/cups/ssl’: Permission denied
find: ‘/etc/cni/net.d’: Permission denied
find: ‘/etc/nftables’: Permission denied
find: ‘/etc/audit’: Permission denied
find: ‘/etc/sudoers.d’: Permission denied
find: ‘/etc/libvirt’: Permission denied
find: ‘/etc/containers/networks’: Permission denied
find: ‘/etc/sos/cleaner’: Permission denied
find: ‘/etc/ipsec.d’: Permission denied
find: ‘/etc/firewalld’: Permission denied

7、将进程输出保存到/tmp/output文件中，将错误消息保存到/tmp/errors文件中。

[[email protected] ~]$ find /etc -name passwd > /tmp/output 2> /tmp/errors

[[email protected] ~]$ cat /tmp/output
/etc/pam.d/passwd
/etc/passwd

[[email protected] ~]$ cat /tmp/errors
find: ‘/etc/lvm/archive’: Permission denied
find: ‘/etc/lvm/backup’: Permission denied
find: ‘/etc/lvm/cache’: Permission denied
find: ‘/etc/pki/rsyslog’: Permission denied
find: ‘/etc/pki/pesign’: Permission denied
find: ‘/etc/ssh/sshd_config.d’: Permission denied
find: ‘/etc/grub.d’: Permission denied
find: ‘/etc/sssd’: Permission denied
find: ‘/etc/polkit-1/rules.d’: Permission denied
find: ‘/etc/polkit-1/localauthority’: Permission denied
find: ‘/etc/cups/ssl’: Permission denied
find: ‘/etc/cni/net.d’: Permission denied
find: ‘/etc/nftables’: Permission denied
find: ‘/etc/audit’: Permission denied
find: ‘/etc/sudoers.d’: Permission denied
find: ‘/etc/libvirt’: Permission denied
find: ‘/etc/containers/networks’: Permission denied
find: ‘/etc/sos/cleaner’: Permission denied
find: ‘/etc/ipsec.d’: Permission denied
find: ‘/etc/firewalld’: Permission denied

8、忽略并丢弃错误消息，但保存进程输出的消息。

[[email protected] ~]$ find /etc -name passwd > /tmp/export 2> /dev/null

[[email protected] ~]$ cat /tmp/export
/etc/pam.d/passwd
/etc/passwd

[[email protected] ~]$ cat /dev/null

9、将输出和生成的错误消息存储在一起。

[[email protected] ~]$ find /etc -name passwd &> /tmp/save-both

[[email protected] ~]$ cat /tmp/save-both
find: ‘/etc/lvm/archive’: Permission denied
find: ‘/etc/lvm/backup’: Permission denied
find: ‘/etc/lvm/cache’: Permission denied
find: ‘/etc/pki/rsyslog’: Permission denied
find: ‘/etc/pki/pesign’: Permission denied
find: ‘/etc/ssh/sshd_config.d’: Permission denied
/etc/pam.d/passwd
find: ‘/etc/grub.d’: Permission denied
/etc/passwd
find: ‘/etc/sssd’: Permission denied
find: ‘/etc/polkit-1/rules.d’: Permission denied
find: ‘/etc/polkit-1/localauthority’: Permission denied
find: ‘/etc/cups/ssl’: Permission denied
find: ‘/etc/cni/net.d’: Permission denied
find: ‘/etc/nftables’: Permission denied
find: ‘/etc/audit’: Permission denied
find: ‘/etc/sudoers.d’: Permission denied
find: ‘/etc/libvirt’: Permission denied
find: ‘/etc/containers/networks’: Permission denied
find: ‘/etc/sos/cleaner’: Permission denied
find: ‘/etc/ipsec.d’: Permission denied
find: ‘/etc/firewalld’: Permission denied

10、将输出和生成的错误附加到现有文件。

[[email protected] ~]$ find /etc -name passwd >> /tmp/save-both 2>&1

[[email protected] ~]$ cat /tmp/save-both
find: ‘/etc/lvm/archive’: Permission denied
find: ‘/etc/lvm/backup’: Permission denied
find: ‘/etc/lvm/cache’: Permission denied
find: ‘/etc/pki/rsyslog’: Permission denied
find: ‘/etc/pki/pesign’: Permission denied
find: ‘/etc/ssh/sshd_config.d’: Permission denied
/etc/pam.d/passwd
find: ‘/etc/grub.d’: Permission denied
/etc/passwd
find: ‘/etc/sssd’: Permission denied
find: ‘/etc/polkit-1/rules.d’: Permission denied
find: ‘/etc/polkit-1/localauthority’: Permission denied
find: ‘/etc/cups/ssl’: Permission denied
find: ‘/etc/cni/net.d’: Permission denied
find: ‘/etc/nftables’: Permission denied
find: ‘/etc/audit’: Permission denied
find: ‘/etc/sudoers.d’: Permission denied
find: ‘/etc/libvirt’: Permission denied
find: ‘/etc/containers/networks’: Permission denied
find: ‘/etc/sos/cleaner’: Permission denied
find: ‘/etc/ipsec.d’: Permission denied
find: ‘/etc/firewalld’: Permission denied
find: ‘/etc/lvm/archive’: Permission denied
find: ‘/etc/lvm/backup’: Permission denied
find: ‘/etc/lvm/cache’: Permission denied
find: ‘/etc/pki/rsyslog’: Permission denied
find: ‘/etc/pki/pesign’: Permission denied
find: ‘/etc/ssh/sshd_config.d’: Permission denied
/etc/pam.d/passwd
find: ‘/etc/grub.d’: Permission denied
/etc/passwd
find: ‘/etc/sssd’: Permission denied
find: ‘/etc/polkit-1/rules.d’: Permission denied
find: ‘/etc/polkit-1/localauthority’: Permission denied
find: ‘/etc/cups/ssl’: Permission denied
find: ‘/etc/cni/net.d’: Permission denied
find: ‘/etc/nftables’: Permission denied
find: ‘/etc/audit’: Permission denied
find: ‘/etc/sudoers.d’: Permission denied
find: ‘/etc/libvirt’: Permission denied
find: ‘/etc/containers/networks’: Permission denied
find: ‘/etc/sos/cleaner’: Permission denied
find: ‘/etc/ipsec.d’: Permission denied
find: ‘/etc/firewalld’: Permission denied

二、构建管道

说明：管道是一个或者多个命令的序列，用竖线字符“|”分隔；管道将第一个命令的标准输出连接到下一个命令的标准输入。想象一下：数据正在通过管道从一个进程流向另一个进程，在其流过的管道中每个命令都会略微对其做些改动。

重定向：会向文件发送标准输出或从文件获取标准输入。

管道：会将一个进程的标准输出发送到另一个进程的标准输入。

示例

1、使用ls命令的输出并使用less在终端上以一次一次的方式显示输出。

[[email protected] ~]$ ls -l /usr/bin | less

2、ls命令的输出传送到wc -l，用户统计从ls收到的行数，并将该行数显示在终端。

[[email protected] ~]$ ls | wc -l
9

3、在管道中输入出当前查询结果的前10行，并将最终结果重定向到一个文件中。

[[email protected] ~]$ ls -t | head -n 10 > /tmp/ten-last
[[email protected] ~]$ cat /tmp/ten-last
rhel-8.6-x86_64-dvd.iso
Downloads
Desktop
Documents
Music
Pictures
Public
Templates
Videos

三、管道、重定向和tee命令

当重定向与管道组合时，shell会首先设置整个管道，然后重定向输入/输出。如果在管道的中间使用了输出重定向，则输出将转至文件，而不是前往管道中的下一个命令。tee命令克服了这个限制；在管道中，tee将其标准输入复制到其标准输出中，并且还将标准输出重定向到指定为命令参数的文件。如果将数据想象成流经管道的水，那么可将tee视觉化为管道中的T形接头，负责输出在两个方向上的流向。

示例

1、将ls命令的输出重定向到文件，并且将输出传递到less，以便在终端上以一次一屏的方式显示。

[[email protected] ~]$ ls -l | tee /tmp/save-both | less

2、如果管道末尾使用了tee，则可以保存命令的最终输出并且同时输出到终端。

[[email protected] ~]$ ls -t | head -n 10 | tee /tmp/ten-lest
rhel-8.6-x86_64-dvd.iso
Downloads
Desktop
Documents
Music
Pictures
Public
Templates
Videos
[[email protected] ~]$ cat /tmp/ten-lest
rhel-8.6-x86_64-dvd.iso
Downloads
Desktop
Documents
Music
Pictures
Public
Templates
Videos