Bash
- Common
- if
- while / until
- case
- for loop
- select
- continue / break
- built-in commands
- Execution
- 环境变量
- 常用环境变量
- redirection
- pipeline
- 数组操作
- 扩展
  - 大括号扩展
  - 变量扩展
Bash Commands Notes

Bash

Common

;
- bash在解析字符的时候，对待“;”跟看见回车是一样的行为
[ ... ]
- 其实就是个shell test命令的另一种写法
- 命令加参数要用空格分隔, 所以 [ ... ] 需要加空格
[[ ... ]]
- a Bash extension , bash, zsh, yash support it
- bash built-in, and cannot be used in a #!/bin/sh script
- benefits
  - == and != perform pattern matching, so the right-hand side can be a glob pattern
```
$ [[ abc = *bc ]] ; echo $?
0
```
  - =~ performs regular expression matching. Captured groups are stored in the BASH_REMATCH array.
  - boolean operators && and ||
  - no word splitting, so it's not strictly necessary to quote your variables.
- drawback:
  - your script is now bash-specific.
(...)
- indicate a subshell
- What's inside is a list of commands (just like outside parentheses)
- These commands are executed in a separate subprocess,
  - so any redirection, assignment, etc. performed inside the parentheses has NO EFFECT outside the parentheses.
- With a leading dollar sign, $(…) is a command substitution
- the output from the command is used as part of the command line
((...))
- double parentheses surround an arithmetic instruction
- mostly used for assignments and in conditionals. This only exists in ksh/bash/zsh, not in plain sh.
- The same syntax is used in arithmetic expressions $((…)), which expand to the integer value of the expression.
list
- 若干shell命令组成的序列，使用 管道，;，& ，&& ，|| 这些符号串联起来
一切皆表达式
- shell在执行任何东西（注意是任何东西，不仅是命令）的时候都会有一个 return status
- 0-255
  - 0为真（true）
  - 非0为假（false）
- echo $? 来查看上一个执行命令的返回值
!
- 对表达式求反
bash 通配符
- ? 单个字符
- * 任意数量的字符
- [...] 类似正则表达式中的[]
$ 变量
- $0 获得自己执行命令的进程名称
- $n 第n个参数
- $# 参数个数
- $*, $@ holds list of all arguments passed to the script
  - There is no difference if you do NOT put $* or $@ in quotes.
  - But if you put them inside quotes (which you should, as a general good practice),
    - then $@ will pass your parameters as separate parameters,
    - whereas $* will just pass all params as a single parameter.
  - https://stackoverflow.com/questions/22589032/what-is-the-difference-between-and
- $? 上一个命令的返回值
- 所以上述所有取值都可以写成${x}的方式

if

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

等价于

if list
then 
  list
elif list
then 
  list
... 
else 
  list
fi

if 后面跟的就是个shell命令
- return status 作为 condition (0 is true)
- if cd mydir; then ...
if语法中后面最常用的命令就是 []
- if [ $ret -eq 0 ] ...
- 等价于 if test $ret -eq 0 ...

while / until

while list-1
do
    list-2
done

until list-1
do
    list-2
done

while list-1; do list-2; done
until list-1; do list-2; done

case

case EXPRESSION in

  PATTERN_1)
    STATEMENTS
    ;;

  PATTERN_2)
    STATEMENTS
    ;;

  PATTERN_N)
    STATEMENTS
    ;;

  *)
    STATEMENTS
    ;;
esac

for loop

for VARIABLE in LIST
do
    command1
    command2
done

while LIST could be
1. for VARIABLE in 1 2 3 4 5 .. N
2. for VARIABLE in file1 file2 file3
3. for OUTPUT in $(Linux-Or-Unix-Command-Here)
4. for i in {1..5}
  - bash3.0+

C-sytle syntax

for (( initializer; condition; step ))
do
  shell_COMMANDS
done

e.g.

for (( c=1; c<=5; c++ ))
do
   echo "Welcome $c times"
done

select

select i in a b c d
do
    echo $i
done

提供给了我们一个构建交互式菜单程序的方式

$ ./select.sh
1) a
2) b
3) c
4) d
#?


#!/bin/bash
select i in a b c d
do
    case $i in
        a)
        echo "Your choice is a"
        ;;
        b)
        echo "Your choice is b"
        ;;
        c)
        echo "Your choice is c"
        ;;
        d)
        echo "Your choice is d"
        ;;
        *)
        echo "Wrong choice! exit!"
        exit
        ;;
    esac
done

continue / break

内建命令，并不是关键字

built-in commands

source, or .
- 读取文件的内容，并在当前bash环境下将其内容当命令执行
read
- 从标准输入读取输字符串到一个变量中
  - read -p "Login: " username
- 读取到数组中
  - read -a test
mapfile
- 将一个文本文件直接变成一个数组，每行作为数组的一个元素
readarray
- 同上
- 如果内建命令放到管道环境中执行，那么bash会给它创建一个subshell进行处理。于是创建的数组实际上与父进程没有关系
printf
- 进行格式化输出
getopts
- 命令行参数处理

Execution

优先级
- alias
- keyword
- function
- built-in commands (cd, pwd, etc...)
- hash (用hash命令可以查看 hash情况)
- external commands

you can use type to check whether a command is built-in, external, alias, or etc.

$ type pwd
pwd is a shell builtin
$ type docker
docker is /usr/local/bin/docker
$ type myfind
myfind is a shell function from /Users/xxx/.myProfile

脚本的退出
- built-in command exit , 人为指定退出的返回码是多少
- 如果不使用exit 指定，使用最后执行命令的返回码
调试
- -v 可视模式, 执行bash程序的时候将要执行的内容也打印出来
- -x 跟踪模式(xtrace), 跟踪各种语法的调用，并打印出每个命令的输出结果
- -n 检查bash的语法错误, 不会真正执行bash脚本
- -e 脚本命令执行错误的时候直接退出

环境变量

env
- 查看当前bash已经定义的环境变量
aaa=100
- 这是个一般变量，不能被子进程继承
export
- 可以将一个一般变量转成环境变量
- export aaa

常用环境变量

进程信息
- HOSTNAME
- HOSTTYPE
- OLDPWD / PWD
- HOME
- SHELL
- BASHPID
- UID / EUID
- GROUPS 用户组
- PPID 父进程PID
RANDOM
- 得到一个0-32767的随机数
```
echo $RANDOM
24746
```
ulimit
- 查看和设置bash环境中的资源限制
- ulimit -a

redirection

shell在产生一个新进程后，新进程的前三个文件描述符都默认指向三个相关文件
- 0 stdin
- 1 stdout
- 2 stderr
stdin redirection
- 可以读取其他终端输入
- cat < /dev/pts/3
stdout redirection
- >
stderr redirection
- 2>
常见用法
- 只看报错信息
  - xxx > /dev/null
- 只看正确输出
  - xxx 2> /dev/null
- 所有输出都不看
  - xxx &> /dev/null
  - or xxx >& /dev/null
  - short-cut to > /dev/null 2>&1, but not support by Dash, POSIX Shells, and Ksh, e.g. /bin/sh
- 将标准报错输出，重定向到和标准输出相同的地方
  - xxx 2>&1
  - &1 表示引用 fd 1
- 从描述符3读取
  - 3<
To understand "redirecting to /dev/null"
- redirect both stdin and stderr to /dev/null
  - CMD 1> /dev/null 2> /dev/null
- Enhancement 1: You can replace 1> with just >. This is because 1 is the default stdout and you can ignore mentioning defaults.
  - CMD > /dev/null 2> /dev/null
- Enhancement 2: You can replace the 2nd file redirect (> /dev/null) with a file descriptor duplication (>& 1). This is because /dev/null is already pointed to by stdout 1.
  - CMD > /dev/null 2>& 1
- Enhancement 3: This is such a common operation, that many shells have a shortened form of this as a single &> operator.
  - CMD &> /dev/null

pipeline

|
- command1 | command2
- 将command1的stdout跟command2的stdin 通过管道(pipe)连接起来
|&
- command1 |& command2
- 将command1 stdout 和 stderr 都跟command2的和stdin连接起来
- command1 2>&1 | command2 的简写方式

数组操作

定义数组
- declare -a array
元素赋值
- array[0] = 1000
取值
- ${array[0]}
- ${array[*]} (get all)
- ${#array[*]} (数组长度)
定义一个关键数组
- declare -A array

扩展

大括号扩展

$ {a,b,c,d}{1,2,3,4}
a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3 c4 d1 d2 d3 d4

$ {a,c}.conf
a.conf c.conf

变量扩展

:- A or 1000
- ${aaa:-1000}
:= A or A=1000
- ${aaa:=1000}
Slice
- ${aaa:10:5}
  - aaa[10:15]，[start:length]
- ${aaa:10}
  - aaa[10:]
- ${aaa: -5}
  - aaa[:-5] 注意负号前面的空格
- ${#aaa}
  - 变量值的长度
- ${aaa#pattern}
  - remove pattern
  - 从左往右, 从头match 到第一个 ^pattern，删除pattern，取其后面的字串
- ${aaa##pattern}
  - greedy version
  - keey base path
```
$ aaa="/Volumes/WORK/WORK/test"
$ echo ${aaa##*/}
test
```
  - get file extension
```
${aaa##*.}
```
- ${aaa%pattern}
  - also remove pattern, except it apply to the back of variable.
```
$ FILE="xcache-1.3.0.tar.gz"
$ echo "${FILE%.tar.gz}"
xcache-1.3.0
```
  - remove file extension
```
${aaa%.*}
```
- ${aaa%%pattern}
  - greedy version
字符串替换
- ${aaa/pattern/string}
  - 将pattern匹配到的第一个字符串替换成string
- ${aaa//pattern/string}
  - 全局替换
算数扩展
- ((...))
```
$ a=1
$ ((a++))
$ echo $a
2
```
- 支持整数运算/位运算/关系运算/三元运算
- 内建命令let
  - let i=i**2
  - (()) 的另一种写法

Bash Commands Notes

sort / uniq

uniq : report or filter out repeated lines
uniq 多伴随着 sort 命令同时出现, 因为 uniq 只能去掉相邻的重复line
虽然 sort -u 自己就可以去掉重复, 但 uniq 有这更多的功能
uniq -c 打印重复的次数
```
5 chocolate
4 mint
1 ranbow
2 rocky
```
uniq -d
- display only repeated lines
```
chocolate
mint
rocky
```
uniq -u
- display non-dup lines
```
ranbow
```

combo...

$ sort favFlavors.txt | uniq -c | sort -nr
5 chocolate
4 mint
2 rocky
1 ranbow

expansion

$ ls *.{json,py}

$ echo Day{1..7}
Day1 Day2 Day3 Day4 Day5 Day6 Day7

du (disk usage)

disk usage

su (switch user)

$ su testuser

To simulate a full login

$ su -[l] testuser

permission

$ ls -l
drwxr-xr-x  5  mebusy  staff   160 Aug  1  2022 go

drwxr-xr-x

the 1st character could be
1. d : a directory
2. - : a file
3. l : a symbolic link
the following 9 characters are rwx permssions for owner, group, others (any user)

chmod

syntax:

chmod mode file

who
- u : user
- g : group
- o : others
- a : all
what
- - : remove permission
- + : grants permission
- = : set a permission and remove others
which
- r : read permission
- w : write permission
- x : execute permission

examples:

add write permission to group
```
chmod g+w file.txt
```

remove write permission from all

chmod a-w file.txt
# or
chmod -w file.txt

prevent others to access this file
```
chmod o-rwx file.txt
```

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linux_bash_crash.md

linux_bash_crash.md

Bash

Common

if

while / until

case

for loop

select

continue / break

built-in commands

Execution

环境变量

常用环境变量

redirection

pipeline

数组操作

扩展

大括号扩展

变量扩展

Bash Commands Notes

sort / uniq

expansion

du (disk usage)

su (switch user)

permission

chmod

Files

linux_bash_crash.md

Latest commit

History

linux_bash_crash.md

File metadata and controls

Bash

Common

if

while / until

case

for loop

select

continue / break

built-in commands

Execution

环境变量

常用环境变量

redirection

pipeline

数组操作

扩展

大括号扩展

变量扩展

Bash Commands Notes

sort / uniq

expansion

du (disk usage)

su (switch user)

permission

chmod