Linux系统 bash

1. version
bash --version
echo $BASH_VERSION

2. initalize

1. /etc/profile
   The systemwide initialization file, executed for login shells

2. _{/.bash_profile(}/.bash_login, ~/.profile) <= Login shell
   The personal initialization file, executed for login shells
   It is sourced after the /etc/profile. If ~/.bash_profile doesn’t exist, bash will look for another user-defined file, ~./bash_login, and source it, and if ~./bash_login doesn’t exist, it will source the ~/.profile, if it exists. Only one of the three files will be sourced.

3. .bashrc <= interactive shell
   The individual per-interactive-shell startup file
   set ENV variable. This file is automatically sourced every time a new or interactive bash shell or bash script starts.

配置shell
/etc/profile 所有用户环境信息，从/etc/profile.d中的配置文件收集shell设置
/etc/bashrc 运行bash shell的用户执行该文件，用户可在自己的.bashrc中overwrite该文件中设置的环境变量
~/.bash_profile 用户登录时执行一次，默认情况下，设置环境变量并执行.bashrc
~/.bashrc 用户登录或运行bash shell时读取该文件
~/.bash_logout 退出最后一个bash shell时执行

4. /etc/bashrc
   Systemwide functions and aliases.

5. ~/.bash_logout
   The individual login shell cleanup file, executed when a login shell exits

# bash环境配置：

1. login shell，主要读取/etc/profile及~/.bash_profile
2. non-login shell，主要读取~/.bashrc

3. Prompt

PS1='[\u@\h \W]\$'
PS1='[\u@\h \w \A #\#]\$ '

PS2=">"

3. Arithmetic Expansion

echo $[ 5 + 4 * 3 ]		# 17
echo $(( 3 - 7 / 3 ))	# 1

4. Array

declare -a friends=(Sheryl Peter Louise)
echo ${friends[0]}						# Sheryl
echo ${friends[*]}						# Sheryl Peter Louise
echo ${#friends[*]}						# 3

5. Rediretion

Redirection Operator
What It Does
< filename
Redirects input
> filename
Redirects output
>> filename
Appends output
2> filename
Redirects error
2>> filename
Redirects and appends error
&> filename
Redirects output and error
>& filename
Redirects output and error (preferred way)
2>&1
Redirects error to where output is going
1>&2
Redirects output to where error is going
>|
Overrides noclobber when redirecting output
<> filename
Uses file as both standard input and output if a device file (from /dev)

6. read

read answer
read first last
read				# $REPLY
read -a array
read -p prompt
read -r line		# allows the input to contain a backslash

# input in 30s
read -p "please input your name: " -t 30 name

7. Arithmetic

declare -i x=017	# 15
x=2#101			# 5
x=8#17			# 15
x=16#b			# 11

8. test

name=Tom
[ $name = [Tt]?? ]
echo $?				# 1, don't allow wildcard expansion

[[ $name == [Tt]?? ]]
echo $?				# 0

9. Variable

# local variable
declare name="Tom"

# global variable
export ORACLE_SID=ora11g
declare -x ORACLE_SID=ora11g

10. arguments

echo $1 $2 $3
echo $*
echo $#		# the number of arguments

11. array

1. declare
   declare -a array
   array[key]=value
   array=(val1 val2 … valn)
   declare -a fruit=( apples pears plums )

2. access
   ${array[key]}
   ${array[0]} <=> ${array[@]:0:1} <=> $array
   ${array[@]} <=> ${array[*]}
   ${array[@]:0:2} <=> ${array[0]} ${array[1]}
   ${array[@]:2} <=> ${array[2]} … ${array[n]}

3. eliminate
   unset ${array[0]}
   unset array

4. length
   ${#array[@]}
   ${#array} <=> ${#array[0]} # length of first element

12. arithmetic
typeset -i num=5+4
declare -i num=5+4

13. control statement
if (( numeric expression ))
if [[ string expression ]]

while (( numeric expression ))
while [[ string expression ]]

until (( numeric expression ))
until [[ string expression ]]

for name in Tom Dick Harry

PS3=“Select an name from the menu:”
select name in Tom Dick Harry

14. 内置命令和外部命令
type -all pwd
command pwd # 强制将pwd当命令执行

cd //
pwd               # //, here is built-in
/bin/pwd          # /, here is command
enable -n pwd     # 屏蔽built-in command pwd
pwd               # /

15. Quote

hard quote: '', 屏蔽所有meta
soft quote: "", 屏蔽大部分meta，$`\除外
escape: \, 屏蔽其后紧随的meta, & * + ^ $ ` " | ?

~> a="b
> c
> "
~> echo $a    --meta为\n，未在soft quote中，被解释为IFS
b c
~> echo "$a"
b
c

~> a=b\
> c\
> 
~> echo $a   --escape关闭CR
bc
~> echo "$a"
bc

awk '{print $0}'        # hard quote
awk "{print \$0}"       # soft quote
awk \{print\ \$0\}      # escape

A=0
awk "{print \$$A}" 
awk \{print\ \$$A\} 
awk '{print $'$A'}' 
awk '{print $'"$A"'}' 

16. IFS: Internal Field Seperator

~> set | grep IFS
IFS=$' \t\n'

17. null value & unset

str=
var=${str=expr}
echo $var   # null
echo $str   # null

unset str
var=${str=expr}
echo $var   # expr
echo $str   # expr

18. fork, source, exec
fork: 产生子进程，子进程执行完毕，返回父进程。
source：不产生子进程，在当前shell中执行，执行完成后，环境变量可能被改变
exec：It replaces the shell. No new process is created.

19. {}, (), $(()), $(), ${}
(): nested sub-shell
{}: non-named command group

$( ) <=> # 后者在unix shell中移植性较高

${ } 变量替换

${file#*/}      # dir1/dir2/dir3/my.file.txt
${file##*/}     # my.file.txt
${file#*.}      # file.txt
${file##*.}     # txt

${file%/*}      # /dir1/dir2/dir3
${file%%/*}     # null
${file%.*}      # /dir1/dir2/dir3/my.file
${file%%.*}     # /dir1/dir2/dir3/my

• 去掉左边

• % 去掉右边
• 一个最小匹配，两个最大匹配

# 截取字符串
${file:0:5}      # /dir1
${file:5:5}      # /dir2

# 字符串替换
${file/dir/path}     # /path1/dir2/dir3/my.file.txt
${file//dir/path}    # /path1/path2/path3/my.file.txt

# 变量状态 unset, null, non-null
${file-my.file.txt}		# if file unset, my.file.txt
${file:-my.file.txt}	# if file unset/null, my.file.txt
${file+my.file.txt}		# if file null/non-null, my.file.txt
${file:+my.file.txt}	# if file non-null, my.file.txt

${file=my.file.txt}		# if file unset, my.file.txt, and file=my.file.txt
${file:=my.file.txt}	# if file unset/null, my.file.txt, and file=my.file.txt

${file?my.file.txt}		# if file unset, STDERR my.file.txt
${file:?my.file.txt}	# if file unset/null, STDERR my.file.txt

# 字符串长度
${#file}

# 数组长度
${#A[@]}

$(( )) 整数计算
a=5; b=7; c=2
echo $(( $a - $b * $c))  # -9

20. pwd (bash)
pwd -P: 真实路径(等效于set -o physical)
pwd -L: 软连接路径(默认开启)

21. 防止文件被overwrite

set -o noclobber
$ cat file.out 
1
$ set -o noclobber
$ echo 2 > file.out
bash: file.out: cannot overwrite existing file

type: bash built-in
-t: (file, alias, buitin)
-p: path
-a: all

type ls
type -t ls
type -t passwd
type -p passwd
type -a passwd
type cd

子程序继承父程序的环境变量，但不能继承父程序的自定义变量

read -p "Please enter your name: " -t 30 name
echo $name

declare
-a: array
-i: integer
-x: export
-r: readonly

declare -i num=100+200+300


ulimit -a
ulimit -f 10240  # 不能创建超过10M的文件

变量内容删除，替换
${PATH#*/bin:}	# 头端最短删除
${PATH##*/bin:}	# 头端最长删除
${PATH%:*bin}	# 尾端最短删除
${PATH%%:*bin}	# 尾端最长删除

a="123abc123"
echo ${a/123/456} 	# 456abc123
echo ${a//123/456}	# 456abc456

# 登录提示信息
/etc/motd

# bash配置
login shell: 由tty1~tty6登录，需要输入用户名和密码，此时取得的bash为login shell
non-login shell: 不需要登录就能取得的bash。1) X window登录后，启动的终端 2) 子程序

1) /etc/profile (login shell才会读)
自动呼叫/etc/inputrc; /etc/profile.d/*.sh; /etc/sysconfig/i18n

2) ~/.bash_profile(login shell才会读)
 ~/.bash_profile
 ~/.bash_login
 ~/.profile
bash依顺序读取上面三个文件，允许文件不存在


3) ~/.bashrc (non-login shell会读)

stty: setting tty
stty erase ^h  # ctrl + h 删除
stty erase ^? # Backspace 删除

# $-: set配置值
echo $-		# himBH


cut -d'delimeter' -f fields
cut -c characters

echo $PATH | cut -d ":" -f 2,4-6 
export | cut -c 12- 

cat /etc/passwd | sort -t ":" -k 3
cat /etc/passwd | sort -t ":" -k 3 -n


last | tr '[a-z]' '[A-Z]'
cat /etc/passwd | tr -d ":"
cat dos.txt | tr -d '\r'

col -x:  output multiple spaces instead of tabs
cat /etc/manpath.config | col -x | cat -A

col -b: do not ouput any backspaces
man col | col -b > /tmp/col.man

head -3 /etc/passwd /etc/shadow
join -t ":" /etc/passwd /etc/shadow | head -3

# 根据group ID 链接
join -t ":" -1 4 /etc/passwd -2 3 /etc/group

# 文件行合并，分隔符TAB，其他分割符使用-d指定
paste /etc/passwd /etc/shadow

# TAB转换为空格，默认8个
grep '^MANPATH' /etc/manpath.config | head -3 | cat -A
grep '^MANPATH' /etc/manpath.config | head -3 | expand -t 4 | cat -A

# split
split -b 1m myfile.txt myfile