linux shell & bash

shell & bash

shell指允许用户通过文本操作计算机的程序。

interactive shell：从是否通过标准输入输出与用户进行交互的角度分为交互式shell（interactive）和非交互式shell（non-interactive）。

login shell：从是否以一个指定用户及其环境进入shell角度分为登录式shell和非登录式shell，登录式shell会额外source /etc/profile,/etc/profile.d,~/.profile,~/.bash_login等profile相关脚本。

变量$-包含了当前shell的选项参数，一个字母代表一种选项。部分如下：

• H histexpand
• m monitor
• h hashall
• B braceexpand
• i interactive

判断是否是交互式shell：根据$-输出是否包含字母i来判断。

case "$-" in
*i*)	echo This shell is interactive ;;
*)	    echo This shell is not interactive ;;
esac

if [[ "$-" == *i* ]]; then
    echo This shell is interactive
fi

当前使用的shell类型（bash, cshell...）：echo \(0；注意\)SHELL是默认shell，并非当前shell，也就说如果在echo $SHELL不是当前shell的类型。

uniq只会删除连续重复的行，也就说uniq不在输入的所有行上做distinct，只是在剪掉连续出现的行。想要对在输入的所有行上做distinct，可以考虑先sort，然后uniq，但需要注意这样会改变输入中行字符串出现的顺序。

iostat查看系统io，iftop查看网络流的情况。

ssh连接在很短的不活跃时间后自动断开，如需延长这个闲置时间，通过在ssh配置文件中修改ServerAliveInterval ServerAliveCountMax两个量实现，前者指给远程发送心跳包的时间间隔，后者指连续发送心跳包的最大次数。

~/.ssh/config或/etc/ssh/ssh_config中添加两行：

# 每60秒发送一个心跳包
ServerAliveInterval 60
# 最多连续发送3次
ServerAliveCountMax 3

ssh命令行参数提供密码：

sshpass -p PASSWORD ssh -p PORT USER@HOST

sudo -E 保留当前环境变量执行命令

sudo -i 登入root用户shell

查看当前使用的shell：

echo $SHELL

查看系统可用的shel：

cat /etc/shells

更改用户默认的shell：

修改/etc/passwd文件对应用户最后的shell配置。

bash相关配置文件

登录bash shell相关（ubuntu）：/etc/profile -> /etc/bash.bashrc（在/etc/profile中被source） -> /etc/profile.d/ -> ~/.bash_profile -> ~/.bash_login -> .bashrc

交互式bash shell： /etc/bash.bashrc -> .bashrc

如果shell不是bash，/etc/profile中不会source /etc/bash.bashrc，如启动intellij idea IDE时。

shell script脚本

shell脚本（shell script）是指由一系列shell命令组成的脚本程序文件。shell script不是interative shell，也不是login shell。

小心ls、grep等带有颜色的输出，输出字符串看起来一样的，在做uni时被视为不一样，则小心其中有个输出可能带有颜色字符，用cat -A确认。

for, while中计数

counter=0
for/while...
  counter=$((counter+1))

定义环境变量：

export name=value

取消定义：

交换文件中以逗号分割的前两列的纯bash脚本实现：

while IFS=, read x1 x2 remainning
do
   echo ${x2},${x1},$remainning
done < FILE

$PS1中的特殊符号：

\u：用户名

\h：短主机名

\W：working directory(basename)

\s: shell名字（bash、sh）

\v: shell version

\d: 英文日期

\D{fmt}：日期格式（man strftime）

\n: 换行（多行提示符相关）

\w: full path of working directory

\H: full hostname

以及颜色标签[\exxx]

如果不存在则创建目录

[ -d /path/to/dir ] || mkdir -p /path/to/dir

io重定向 io redirect:

stdout stderr输出到同一个文件：

ls -l /usr > out 2>&1 或者 ls -l /usr &> out。

stdout stderr输出到单独文件 ls -l /usr > stdout 2> stderr。

忽略stderr输出 ls -l /usr > stdout 2>/dev/null

date

将epoc seconds格式化输出 date -d @1305741420。注意，需带@前缀。 若想将millis（毫秒）格式化输出则先去掉数值后3位：echo 1305741420 | cut -b1-10 | xargs -I {} date -d @{}

date +"格式化模式"
date +"%Y%m%d-%H%M%S"   # 20470801-050903   2047年8月1日5点9分3秒

格式化：

• %Y 年数，四位数。
• %m 月数，两个字符，00..12，不足时以0补。 （month of year）
• %d 月中的天数（day of month），00..31，两个字符，不足时以0补。
• %H 小时，（01..23），两个字符，不足时以0补。
• %M 分钟数，(01..59)，两个字符，不足时以0补。

xargs

将xargs的输入（通常是前面管道的输出）作为待执行命令最后一个参数，如果需要拼接固定（或其他形式格式化），可以指定替换串形式，如：echo 1305741420 | xargs -I {} | date -d @{}，则指定{}为替换串，会将命令形式date -d @{}中的{}替换为xargs接收到的输入。

awk

行处理，根据列是否满足条件输出、或做进一步处理，这样的需求完全可以考虑使用awk。

忽略#开头的行 awk '$1 ~ /^[^#]/'

忽略#开头的行，输出第三列（默认以空格分隔）以<开头的行awk '$1 ~ /^[^#]/ && $3 ~ /^</'

设置分隔符为逗号,：awk -F, awk 'BEGIN{FS=,}'

sed

#移除文件前n行
sed -i '5d' FILE    # -i 将改动保存到该文件；  5d 删除文件前5行
#移除文件末行
sed -i '$d' FILE    # '$'：最后一行

comm

找出已排序的两个文件中的相同行、不同行，差集、交集、并集皆可实现。

输出3列，分别是：FILE1中独有行、FILE2中独有行、共有行。

comm接受-参数，表示不输出第列，可以是多个列

comm -12仅输出两文件共有行

comm -23仅输出FILE1独有行

comm -13仅输出FILE2独有行

shuf

shuffle lines

shuf <FILE>

... | shuf

join

合并已排序的两个文件中的行。

top

• -b 输出到文件
• -p 指定pid, -pN1,N2,N3
• -O 打印可以输出的字段，相当于对-o的帮助信息
• -n 迭代次数 -n num
• -O 排序字段 [+|-]fieldName

lsblk

分辨ssd/hdd：lsblk -d -o name,rota 输出0代表ssd，1代表hdd。

lsblk -o name可展示出物理盘的从属虚拟硬盘（slave disk），如物理盘虚拟出的lvm盘。

strace

跟踪系统调用和信号。

#查看ping命令的调用过程
strace -e trace=open -f ping -c1 localhost

dhclient

与DHCP服务器协商客户机可使用的IP，以及DNS名字服务器。

dig

DNS查询工具。

#指定DNS服务器，通过 '@server-ip'
dig @127.0.1.1

迭代文件中的行

cat 默认会以空格分割字符串，因此如果文件中行包含空格，则cat不能迭代成功，解决方案是在cat文件之前把IFS变量设为换行符，然后用cat迭代，完成后应该把IFS重置为之前的值

对稍大一点（上百M）的文件，速度相当慢。

IFS=$'\n'
for line in `cat file`
do
    ...
done

算术运算

\(((<表达式>))、\)[<表达式>]、expr <表达式>，其中"<表达式>"为算术运算表达式，这几种形式都只能进行整数运算，不支持浮点数运算。

命令bc和awk支持浮点数运算。

bc FILE对算术表达式文件执行计算，bc -i以交互式进行执行。

控制流程：

if ... ; then
    ...
elif ... ; then
    ...
else
    ...
fi

for x in ...; do
    echo $x
done

# inline
for x in ...; do CMD1; CMD2; done

#文件不存在
if [ ! -f FILE_PATH ]

#文件夹存在
if [ -d DIR_PATH ]

function

#定义函数，关键字function是可不写
function fun(){
    #获取参数
    echo $1 , $2;
    #返回值，可选
    return $(($1 + $2))
}
#调用函数，需在此位置前先定义
fun 5 7;
#获取函数返回值
s = $?

输出错误信息到stderr： echo .... >&2

字符串 String

判断字符串相等、不等：

#相等
if [[ "$a" == "$b" ]]

#不等
if [[ "$a" != "$b" ]]

判断字符串是否包含给定字符串：

if [[ <string> == *"sub-string"* ]]; then
    ...
fi

String Manipulation

online doc

Length

${#str}     #
lengthexpr length $str  #

Index

expr index $str $substr     #

Substring Extraction

${str:position}
${str:position:length}
expr substr $str $position $length

Substring Removal

${string#substring}     # deletes *shortest* match of $substr from *front* of $str
${str##substr}          # deletes *longest* match of $substr from *front* of $str
${str%substr}           # deletes *shortest* match of $substr from *back* of $str
${str%%substr}          # deletes *longest* match of $substr from *back* of $str

Patterns are supported in substr, e.g. ${filename%.jp[e]g} to strip trailing .jpg or .jpeg.

Substitution/Replacement

${str/substr/replacement}   # replace first match
${str//substr/repl}         # replace all matches
${str/#substr/repl}         # if substr matces *front* end of str, substitute repl for substr
${str/%substr/repl}         # if substr matces *back* end of str, substitute repl for substr

下载

wget

根据url下载小文件 wget -nd URL。选项-nd指明不创建目录，否则程序将根据url创建层级目录；选项-b指明后台下载；选项-d打印处理过程；-v指明更详细输出。

定时任务

编写crontab文件

crontab <crontab-file>
crontabl -l # 列出任务

linux文件权限rwx

文件：

目录：

重新扫描硬件：

echo 1 > /sys/block/sd<x>/device/delete
lspci
# rescan hard disk
# where <n> is the BUS number  (the number before the first ':' in text info listed by command 'lspci'
echo "0 0 0" > /sys/class/scsi_host/host<n>/scan

连接其他电脑

ssh ...
#在ssh中
export DISPLAY=:0
[sudo] notify-send <msg>  #右上角显示提示信息
[sudo] xmessage <msg> [-timeout <n>(单位：sec)] [-buttons OK,Cancle] #以xmessage丑陋的窗口弹窗显示信息

#某些可以从命令行打开GUI的程序，如redis-desktop-manager
...

添加路由时报错：

route add -net 172.17.0.0/16 eth0

报错：-net: Unknown host。

路由类型：

• unreachable these destinations are unreachable. Packets are discarded and the ICMP message host unreachable is generated. The local senders get an EHOSTUNREACH error.
• blackhole these destinations are unreachable. Packets are discarded silently. The local senders get an EINVAL error.
• prohibit these destinations are unreachable. Packets are discarded and the ICMP message communication administratively prohibited is generated. The local senders get an EACCES error.

添加命令

ip route add blackhole 10.192.168.0/24

代码注释 Comments

行内注释，以井号#打头。

块注释（多行注释），没有对块注释直接的原生支持，可以利用here-document语法间接实现块注释。

# this is a comment
echo 'balabala'     # comment here

:<<'
block comment, within which there is no single quotation mark
'

.inputrc

# 命令行补全时不区分大小写
set completion-ignore-case on

#设置快捷键
# Alt+h 光标回移一字符
"\eh": backward-char
# Alt + l(小写L）光标前移一字符
"\el": forward-char
"\ej": next-history
"\ek": previous-history
# Alt+Shift+H
"\eH": backward-word
"\eL": forward-word
#行首
"\eJ": beginning-of-line
#行尾
"\eK": end-of-line

命令及功能：

# show full command line for a process
cat /proc/{PID}/cmdline

# sort 排序
sort -h #对 *KB, *MB, *GB格式的数据排序
 -r 逆序（默认升序）
 -n 视为数值（默认视为普通文本）

#打印或设置用户限制
ulimit -a   #打印所有限制
ulimit -n   #打印允许用户可持有打开中的文件的最大数量
ulimit -n <NUMBER>  #设置

#打开socket并往里写数据
nc -k -l <HOST> <PORT>
nc -k -l <PORT>
# -l指定网口、端口，亦可换之分开以 -s <HOST>、 -p <PORT>指定

#查看路由表
route   # -n 显示ip而非主机名
#添加路由规则
route add -net 172.17.0.0/16 dev eth0
#删除路由规则
route del -net 172.17.0.0/16
netstat -rn
ip route
ip route list

#列举文件，不排序。在文件非常多时很实用。
ls -U

#paste ： merge/reduce lines. opts:  -s串联成一串  -d LIST 利用LIST中的字符合并
paste -sd+ FILE | bc  # 将FILE中每行的数字串联成n1+n2+...的形式，输出给命令bc以求和

#测量命令执行的real/cpu/user time
time COMMAND

#将输出保存到文件同时在控制台打印
COMMAND | tee FILE1 [FILE2 ...]   (-a追加)
#重定向std out到FILE1， std err到FILE2，同时在控制台输出std err
COMMAND > FILE1 2> >(tee FILE2 >&2)

#split file at line number
split -l N FILE  #将文件FILE以每N行切分成多个小文件，生成的文件顺序名是aa-zz
split -l N FILE  PREFIX     #定义小文件的文件名前缀（默认'x'）

[ -d "~" ] is false
[ -d ~ ] is true
[ -d "$HOME" ] is true
apt-cache show app # check whether app is installed
apt-get update # update apt repository

telnet <CTRL+]> back to telnet then quit or <CTRL+D> to quit telnet

tail -n +NUM FILE # show line NUM to end

export NAME=xxx
unset NAME

# detect encoding of tile content  检测文件内容编码
enca FILE
# convert encoding of file content
enconv FILE
# convert encoding of text
iconv -f from-encoding -t to-encoding
# convert encoding of file name  文件名
convmv -f from-encoding -t to-encoding --notest
# convert file content between character sets  文件内容
recode FROM-ENC..TO-ENC FILE    (e.g. recode gbk..utf8 /a.txt)
# list the supported character sets of recode
recode -l

lsusb
lscpu

# specify an encoding before extracting
unzip -O encoding
unzip -O cp936 -l chap04.zip | head -n -2 | sed -n '4,$p' | awk '{print $4}' | grep
# read the contents of files in a zip file; extract file into stdout
unzip -c ZIPFILE ENTRY

locate:
 locate -b(basename) xx ; locate -r regex

# manual for command
man xx; man [1-7] xx; man 7 man
# help for shell-built command
help <CMD> # help cd; help pwd
# document
info xx
# search man page names and descriptions
apropos xx

# block device, uuid, &...
(sudo) blkid /device...  ;  blkid -s UUID /device ;  blkid -s LABEL /device

# To cut out the third field of text from file or stdin, which is delimited by a #:
cut -d # -f3
# first char of column instead of whole column, cut twice
cut -d # -f3 | cut -c 1  # cannot cut -f3 -c 1

# sort file name numerically
ls /dev/sda* | sort -k 1.9n
# list files sorted by size(desending order)
ls -S
# list files sorted by time(desending order)
ls -t

# list files, dereference symbolic link
ls -L ...

# short desciption of ascii characters
man 7 ascii

# directory or file usage
du -hs FILE
du -a <directory> # including files not only directory
du -d <depth> FILE 

# disk usage
df -lh /mount/point /# or mounted /dev/xxx, if /dev/xx not mounted, output is not what we expected

# disk ssd or hdd
lsblk -d -o name,rota  # 0 for ssd, 1 for hdd
cat /sys/block/sdx/quene/rotational

# create an iso image file from a cd/dvd/bd
dd if=/dev/cdrom of=/path/to/image.iso

# eject a disc
eject	eject cdrom    eject /mount/point    eject /dev/cdrom

# mount Windows shared folder, mount samba share (cifs <- sambfs)
mount -t cifs -o username="WinUser",password="secret",uid="localuser",gid="localgroup",dir_mode=0777 //192.168.x.x /mnt/win
# maybe need run the following command before mount
apt install cifs-utils # if no cifs filesystem supporting lib in system (by checking if /sbin/mount.cifs exists)

# mount an iso image file
mount -o loop /path/to/image.iso /path/to/mount/point
# mount partition by non-superuser
gvfs-mount -d /dev/sda1 /mnt/point/	# gvfs-* for gnome user only
# unmount
gvfs-mount -u <location>
# list
gvfs-mount -l

无密码访问远程主机R
# generate key pair on local host
ssh-keygen -t rsa
#copy public key id_rsa.pub into remote host R
scp x.pub user@host:/home/u/.ssh/
# on remote host
cat x.pub >> .ssh/authrized_keys

# cgroup: control group
# sudo apt-get install cgroup-bin

# share a read-only/readable-writable directory over lan
?????

# find devices on lan
????

# find a device on lan then mount
?????

# list open files for a process
ls /proc/PID/fd
lsof -p PID

# determine / identify processes using files or sockets
fuser name...

# merge two folders
rsync -abviuzP src/ dest/
    -i turns on the itemized format, which shows more information than the default format
    -b makes rsync backup files that exist in both folders, appending ~ to the old file. You can control this suffix with --suffix .suf
    -u makes rsync transfer skip files which are newer in dest than in src
    -z turns on compression, which is useful when transferring easily-compressible files over slow links
    -P turns on --partial and --progress
    --partial makes rsync keep partially transferred files if the transfer is interrupted
    --progress shows a progress bar for each transfer, useful if you transfer big files

# merge two folders with cp, tip: hard link: cp -r --link dir1/* dir2/* merged/ && rm -r dir1/ dir2/
# IMPOSSIBLE: create necessary destination directories when copying file, e.g. cp foo.txt /path/not/existing/foo.txt

# telnet to a ssl port(e.g. 443)
openssl s_client -connect host:port

# shared library dependencies
ldd BINARY

# list symbols from object files
nm OBJ(?.o) LIB(?.so ?.a)

# set shared library search paths(directories), add paths to /etc/ld.so.conf, then run ldconfig
vim /etc/ld.so.conf
ldconfig
# report missing objects
ldd -r /path/to/ELF_binary
ldd /path/to/executable | grep 'not found'

#
readelf -d <binary> | grep RPATH

# restore trash that deleted as root
su
cd /path/to/parent/of/file/trashed
restore-trash

# change label of partitions
# mlabel or fatlabel for fat32
# ntfslabel for ntfs
# exfatlabel for exFAT
# e2label for ext2/ext3/ext4
# btrfs for BTRFS

# change label of a fat32 filesystem
sudo mlabel -i /dev/sdb1 ::"new_label"
#if fails & print '... add mtools_skip_check=1 to .mtoolsrc', do
#echo "mtools_skip_check=1" >> ~/.mtoolsrc # as the program says
# get or set label of ms-dos filesystem
sudo fatlabel /dev/sdc1 [new_label]
# change label of a ntfs filesystem
sudo ntfslabel /dev/sda7 "new_label"
# change label of a exfat filesystem
sudo exfatlabel /dev/sdc1 "new_label"
# change label of a ext2/3/4 filesystem
sudo e2label /dev/sdc1 "new_label"

# list background jobs
jobs
bg
# foreground jobs
fg

# get gpu information
lspci -v | grep VGA -A 12

# grep
grep PATTERN {FILE | stdin}
 -r # deep in FILE recursively
 -i # case-insensitive
 -v # not matching
 -A NUM, --after-context=NUM # print NUM lines after matching lines
 -B NUM, --before-context=NUM # print NUM lines before matching lines
 -C NUM, --context=NUM
 -H, --with-filename
 -h, --no-filename

sudo ssldump -i lo port 443 

# delete entries from a zip file
zip -d foo.zip foo/entry/to/delete
# delete a directory from a zip file
zip -d foo.zip "foo/dir/*"

# command information
type <command>
# locate a command
which <command> 

# make naming fifo
mkfifo
# make special files
mknod <name> <type>

# change password of current user
passwd
# change another user
sudo passwd <username>

# add group
sudo addgroup <group-name>
# add user
sudo adduser -ingroup <group-name> <user-name>
adduser -g <gid/group-name> -m(--create-home) -p(--password) PASSWORD -s(--shell) SHELL -u(--uid指定UID) UID <user-name>
adduser -U(创建同名组，与-g互斥) <user-name> 

#改变默认shell
chsh -s SHELL [user-name]  #不提供user-name时表示当前用户

# login as / become another user, default as root
su - <user-name>

# tell login shell or not
shopt login_shell

# add user in sudoers
# just put the user into 'sudo' group
usermod -a -G sudo <USER-NAME>  #centos不能
su -		# login as root
visudo
# then move to the last line, write <user-name> ALL=(ALL:ALL) ALL  then ctrl-X to exit, Alt-B to backup previous version file

# screen resolution
xdpyinfo | grep dimensions

# groups of user
groups <user-name>...
# list groups of current user
groups

# add user into group
usermod -a -G <group-name> <user-name>	#e.g usermod -a -G sudo jack

# top
# E : memory usage summary value scale (circle in KB<-->PB)
# ? : memory usage of process: swap between size/percentage: ?
# M : sort by memory usage
# P : sort by cpu usage
# c : toggle command/command-line

# memory usage
free -m (by M unit)
free -g (by G unit)

ps aux -p <PID>

screen [-S session-name] # create a screen sesion
# run some long-time-consuming script
screen -d [session-name] # <Ctrl-A d> detach from session
screen -ls # list sessions
screen -r <session-name> # attach to session
screen -d -r <session-name> # attach , detach first if necessary
# which sesseion i am in
echo $STY
# exit: close session then exit

# to leave the process running after closing the terminal
<ctrl+z> # suspend the process and back to terminal 将当前前台程序挂起后回到终端命令行，被挂起的程序不会得到CPU因此看起来没有工作
jobs  # 列举后台程序（包括被挂起的） show background jobs, find out the job id of corresponding process
bg  # 将最近被挂起的程序以后台方式开始执行
bg %<jobid> # bg %3 第<jobid>号程序后台执行
fg  #将最近的后台程序拿到终端前台执行
fg %<jobid>
disown -h %<jobib>  <- let the job run in background

pgrep -f <pattern> # search pattern in command line (not only in command)
pgrep -a <pattern> # print full command

# find
# find files whose name ends with .c in (and desending into) current directory
find . -name "*.c" # quotes are must. or: find . -name \*.c  or: find .-name '*.c'
# 在大范围内查找（如根目录/）是很鲁莽的，find在大范围内查找的开销较大

#!!! * in a shell command line will be expanded as file names(hidden files excluded), if it is a wildcard in an argument, should be enclosed by qutoes or escaped
# find . -name *.c => means find . -name x.c y.c z.c  where x.c y.c z.c are only files ending with .c in current directory
# find . -name \*.c or "*.c" '*.c' => means find files in .(and desending), which name ending with .c
# find files on which no read-permission
find <FILE> ! -perm -u+r # user-non-readable
find <FILE> ! -perm -g+r # no permision on group
# find files by size
find . -size +100M # larger than 100M
find . -size -100M # smaller than 100M
# find files by owner
find . -user <USER> # username or user id all allowed
# find files by time in day unit, -ctime, -atime, -mtime;  c: change(attributes been changed), a: access, m: modify
# by time in minute unit, -cmin, -amin, -mmin
# time argument, +<NUM> over NUM day/min, -<NUM> within NUM day/min
find . -ctime +2 -and -mmin 60 #
# striping leading './' of output of find .   ; find * will not search hidden file/dir(file of name starts with '.')
find -printf '%P\n'

# find AND
find expr1 -and expr2  #same as expr1 expr2
# find OR
find \(expr1 -or expr2 \)  #没有括号将导致非预期效果

# find no hidden files
find . -not -path '*/\.*'

#
find ...  -exec bash -c '<expression>' \;

# md5 checksum
md5sum FILE...

# gui sudo
gksudo <cmd line>

# print http headers
curl -v <URL>

# startup services management
sysv-rc-conf

# change owner of symbolic file, put -h (no dereference) option
chown -h user:group FILE

# mount remote directory conneting by ssh
sshfs user@host:/path /local/path
# umount remote directory
fusermount -u /local/path

# determine linux distribution info
ls /etc/*-release
# or
lsb_release -a

# print kernal version
uname -a

# print listening, established connections with specific port
(sudo) lsof -i:<PORT> # lsof -i:8080
(sudo) netstat -tunlp|grep <PORT> # -tunlp: tcp, udp

# list symbols in a shared library
readelf -s /path/to/libx.s

# encode a image or a file in base64
base64 <FILE>

# system time(date)
date
# hardware clock time
hwclock   # sudo hwclock

# dmesg, messages from kernel
dmesg

# awk
# swap columns
awk '{print $2"\t"$1}' <FILE>
# accumulate numbers, one per line (sum)
awk '{s+=$1} END {print $s}' <FILE>
# using shell variable in awk, you cannot use one directly
awk -v home=$HOME 'print(home)'  # careful: it's home, not $home
# output columns satisfy criteria, e.g not begin with #, and 3rd column starts with <
awk '$1 ~ /^[^#]/ && $3 ~ /^</'

# create directory if not exist
[ -d /dir ] || mkdir -p /dir

ss -tlnp

# rename network interface temporarily, 'eth0' -> 'eth1'
ifconfig eth0 down
ip link set eth0 name eth1
ifconfig eth1 up
# rename network interface permanently, edit /etc/udev/rules.d/some-file

# io statistics
iostat
iostat -x 2 5 # extented info, update every 2sec, total 5 updates

# top io wait process
iotop

# list open files
lsof -p <pid>

# copy into gui clipboard
xsel -b

# compare unique file line by line, make lines *sorted* & *unique* in each file
comm FILE1 FILE2   # a hyphen - means stdin, only one hyphen allowed
 -1 supress column 1 - unique lines in FILE1
 -2 supress column 2 - unique lines in FILE2
 -3 supress column 3 - unique lines in FILE1 & FILE2
 -12 tells intersect, -23 tells {FILE1} - {FILE2}, -13 tells {FILE2} - {FILE1}

# file status
stat FILE

# system uptime
uptime

# network monitor
iftop

# change user name
usermod -l new_name user_name
# move home dir, -m -d
usermod -m(move) -d <NEW_DIR> <USER>
# change uid
usermod -u <NEW_UID> <USER>
# append user to group, add group to user, -a -G
usermod -a -G <GROUP,GROUP2...> <USER>
# list user and group id
id
# list id and belonging groups
id <USER>
#change group name
groupmod -n <NEW_GP> <GROUP>

#算术运算
# $[...] ; $((...)) ; expr ... ; 功能、方式相同，且均只支持整数运算，不支持浮点
$[1+2]  # $((1+2))
$[1*2]
# bc, awk命令支持浮点
echo 'scale=3; 1/3' | bc    # 设置精度为3，计算1/3的结果，必须设置精度，否则为整数运算（即使表达式为1.0/3）
echo '1 3' | awk 'printf(%0.3f\n,$1/$2)'

#查看cpu温度
#安装
sudo apt-get install lm-sensors -y
sudo sensors-detect     #初始化传感器检测。一直按回车使用选项
# 检测温度
sensors