记录常忘记的Linux命令

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##8.备份树莓派SD卡系统##
边备份边压缩
sudo dd if=/dev/sdb | gzip>/home/xxx/xxx.gz
备份到镜像文件
sudo dd if=/dev/sdb of=/home/xxx/xxx.img
从镜像文件中恢复
sudo dd if=/home/xxx/xxx.img of=/dev/sdb

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##7.快速清空文件内容##
使用>filename。
>将输出重定向到文件，如果文件不存在就创建文件；如果文件存在就覆盖文件。
此处>前面不带输出，即用“空”去覆盖文件，也就清空了文件内容。

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##6.解压缩GBK/GB2312编码ZIP文件##
由于zip格式没有指定编码格式，Windows下的编码格式为GBK/GB2312等，Linux下默认编码格式为UTF8，故有些zip文件在Linux下解压时会出现乱码问题。
使用unzip -O cp936 xxx.zip指定字符编码则能正常解压缩。
-O CHARSET为DOS、Windows和OS/2 生成的压缩文件指定字符编码。
cp936即GBK，IBM在发明Code Page的时候将GBK放在第936页。

[260174@w26-260174 Senesors]$ unzip
UnZip 6.00 of 20 April 2009, by Info-ZIP.  Maintained by C. Spieler.  Send
bug reports using http://www.info-zip.org/zip-bug.html; see README for details.

Usage: unzip [-Z] [-opts[modifiers]] file[.zip] [list] [-x xlist] [-d exdir]
  Default action is to extract files in list, except those in xlist, to exdir;
  file[.zip] may be a wildcard.  -Z => ZipInfo mode ("unzip -Z" for usage).

  -p  extract files to pipe, no messages     -l  list files (short format)
  -f  freshen existing files, create none    -t  test compressed archive data
  -u  update files, create if necessary      -z  display archive comment only
  -v  list verbosely/show version info       -T  timestamp archive to latest
  -x  exclude files that follow (in xlist)   -d  extract files into exdir
modifiers:
  -n  never overwrite existing files         -q  quiet mode (-qq => quieter)
  -o  overwrite files WITHOUT prompting      -a  auto-convert any text files
  -j  junk paths (do not make directories)   -aa treat ALL files as text
  -U  use escapes for all non-ASCII Unicode  -UU ignore any Unicode fields
  -C  match filenames case-insensitively     -L  make (some) names lowercase
  -X  restore UID/GID info                   -V  retain VMS version numbers
  -K  keep setuid/setgid/tacky permissions   -M  pipe through "more" pager
  -O CHARSET  specify a character encoding for DOS, Windows and OS/2 archives
  -I CHARSET  specify a character encoding for UNIX and other archives

See "unzip -hh" or unzip.txt for more help.  Examples:
  unzip data1 -x joe   => extract all files except joe from zipfile data1.zip
  unzip -p foo | more  => send contents of foo.zip via pipe into program more
  unzip -fo foo ReadMe => quietly replace existing ReadMe if archive file newer

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##5.关闭SSH连接##
先使用w或who查看当前所有连接，再使用who am i查看自己的连接，最后使用pkill -kill -t pts/x关闭不需要的ssh连接。

[lch@test ~]$ who
lch      pts/0        2017-11-01 10:17 (x.x.x.x)
lhq      pts/1        2017-10-31 17:01 (172.27.15.17)
yzx      pts/2        2017-11-01 07:34 (172.27.15.21)
lhq      pts/3        2017-11-01 10:20 (172.27.15.17)
lch      pts/4        2017-11-01 10:22 (x.x.x.x)
[lch@test ~]$ who am i 
lch      pts/0        2017-11-01 10:17 (x.x.x.x)
[lch@test ~]$ pkill -kill -t pts/4
[lch@test ~]$ who
lch      pts/0        2017-11-01 10:17 (x.x.x.x)
lhq      pts/1        2017-10-31 17:01 (172.27.15.17)
yzx      pts/2        2017-11-01 07:34 (172.27.15.21)
lhq      pts/3        2017-11-01 10:20 (172.27.15.17)

#ssh
sudo apt install openssh-server
sudo /etc/init.d/ssh start

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##4. 重启网络##
修改配置文件后重启网络用service network restart。
使用ifconfig查看仍未获取ip地址，查看/etc/sysconfig/network-scripts/ifcfg-ethxxx文件中ONBOOT和BOOTPROTO属性值，ONBOOT=yes表示在系统启动时激活网卡，BOOTPROTO=dhcp表示使用dhcp服务自动获取ip地址，BOOTPROTO=static表示使用静态ip地址，BOOTPROTO=none表示未指定协议。

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##3. CentOS 7 启动界面选择##
不再修改/etc/inittab文件里的runlevel。
默认图形界面：systemctl set-default graphical.target
字符（命令行）界面：systemctl set-default multi-user.target
/etc/inittab文件内容如下：

# inittab is no longer used when using systemd. 
# 
# ADDING CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM. 
# 
# Ctrl-Alt-Delete is handled by /usr/lib/systemd/system/ctrl-alt-del.target 
# 
# systemd uses 'targets' instead of runlevels. By default, there are two main targets: 
# 
# multi-user.target: analogous to runlevel 3 
# graphical.target: analogous to runlevel 5 
# 
# To view current default target, run: 
# systemctl get-default 
# 
# To set a default target, run: 
# systemctl set-default TARGET.target 

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##2. 建立文件/目录链接##
-s为软链接。
链接到目录的话，目录最后需要有/。

root@ubuntu:/# ls -l | grep relearn
root@ubuntu:/# ln -s /mnt/hgfs/linuxc_relearn/ relearn
root@ubuntu:/# ls -l | grep relearn
lrwxrwxrwx   1 root root    25 Jan  4 06:48 relearn -> /mnt/hgfs/linuxc_relearn/
root@ubuntu:/# cd relearn
root@ubuntu:/relearn# ls
1.txt
root@ubuntu:/relearn# 

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##1. 重启ssh服务##
有时ping得通但ssh不上Fedora。

[root@localhost uart_test]# service sshd restart
Redirecting to /bin/systemctl restart  sshd.service
[root@localhost uart_test]#