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。
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 newer5.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)
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。
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 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#
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]#