#!/bin/bash

echo "hello world"

# 添加执行权限
$ chmod +x hello.sh

$ ./hello.sh
hello world

$ cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/bin/dash
/usr/bin/dash
/bin/zsh
/usr/bin/zsh
/usr/bin/tmux

#!/bin/bash
# 这是注释
echo "hello world"

# 有效的
FIRSTLETTERS="ABC"
FIRST_THREE_LETTERS="ABC"
firstThreeLetters="ABC"
MY_SHELL="bash"
my_another_shell="my another shell"
My_Shell="My shell"
_myshell="My shell"

# 无效的
3LETTERS="ABC"
first-three-letters="ABC"
first@Thtree@Letters="ABC"
ABC = "ABC "
MY_SHELL = "bash"
My-SHELL="bash"
1MY_SHELL="My shell"

#!/bin/bash
WORLD="world world"
echo "hello $WORLD"  # hello world world
echo "hello ${WORLD}2" # hello world world2

#!/bin/bash
var="music"
sports='sports'
echo "I like $var"   # I like music
echo "I like ${var}" # I like music
echo I like $var     # I like music
echo 'I like $var'   # I like $var
echo "I like \$var"  # I like $var
echo 'I like \$var'  # I like \$var
echo `bash -version` # GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu)...
echo 'bash -version' # bash -version

#!/bin/bash
echo $SHELL, $USER, $HOME 

# /usr/bin/zsh, razeen, /home/razeen

#!/bin/bash

LIST=$(ls -l)
echo "File information: $LIST"

$ ./info.sh
File information: total 8
-rwxrwxr-x 1 razeen razeen 85 2月   5 07:35 hello.sh
-rwxrwxr-x 1 razeen razeen 58 2月   5 07:36 info.sh

#!/bin/bash
DATE=$(date -u) # UTC 时间#!/bin/bash
DATE=$(date -u) # UTC 时间
WHO=$(whoami) # 用户名
UPTIME=$(uptime) # 系统运行时间
echo "Today is $DATE. You are $WHO. Uptime info: $UPTIME" > logfile
WHO=$(whoami) # 用户名
UPTIME=$(uptime) # 系统运行时间
echo "Today is $DATE. You are $WHO. Uptime info: $UPTIME" > logfile

$ compgen -b | sort
-
.
:
[
alias
autoload
bg
bindkey
break
builtin
bye
cd

$ type cd
cd is a shell builtin

# which sort
/usr/bin/sort

if [ condition-for-test ]
then
  command
  ...
fi

if [ condition-for-test ]; then
  command
  ...
fi

#!/bin/bash
 
VAR=myvar
if [ $VAR = myvar ]; then
    echo "1: \$VAR is $VAR"   # 1: $VAR is myvar
fi
if [ "$VAR" = myvar ]; then
    echo "2: \$VAR is $VAR"   # 2: $VAR is myvar
fi
if [ $VAR = "myvar" ]; then
    echo "3: \$VAR is $VAR"   # 3: $VAR is myvar
fi
if [ "$VAR" = "myvar" ]; then
    echo "4: \$VAR is $VAR"   # 4: $VAR is myvar
fi

#!/bin/bash
VAR=myvar
if [ '$VAR' = 'myvar' ]; then
    echo '5a: $VAR is $VAR'
else
    echo "5b: Not equal."
fibas
# Output:
# 5b: Not equal.

#!/bin/bash

# 这样写就有问题
VAR1="my var"
if [ $VAR1 = "my var" ]; then
    echo "\$VAR1 is $VAR1"
fi
# Output
# error [: too many arguments

# 用双引号
if [ "$VAR1" = "my var" ]; then
    echo "\$VAR1 is $VAR1"
fi

#!/bin/bash
 
VAR2=2
#  由于被识别成一个单词， [] 里面为 true
if [ "$VAR2"=1 ]; then
    echo "$VAR2 is 1."
else
    echo "$VAR2 is not 1."
fi
# Output
# 2 is 1.


# 前后加上空格就好了
if [ "$VAR2" = 1 ]; then
    echo "$VAR2 is 1."
else
    echo "$VAR2 is not 1."
fi
# Output
# 2 is not 1.

# 正确， 符号前后有空格
if [ $VAR2 = 1 ]; then
    echo "\$VAR2 is 1."
else
    echo "It's not 1."
fi
# Output
# 2 is 1.


# 错误， 符号前后无空格
if [$VAR2=1]; then
    echo "$VAR2 is 1."
else
    echo "It's not 1."
fi
# Output
# line 3: =1: command not found
# line 5: [=1]: command not found
# It's not 1.

#!/bin/bash
FILE="/etc/resolv.conf"
if [ -e "$FILE" ]; then
  if [ -f "$FILE" ]; then
      echo "$FILE is a file."
  fi
  if [ -d "$FILE" ]; then
      echo "$FILE is a directory."
  fi
  if [ -r "$FILE" ]; then
      echo "$FILE is readable."
  fi
fi

#!/bin/bash
STRING=""
if [ -z "$STRING" ]; then
  echo "There is no string." >&2 
  exit 1
fi

# Output
# There is no string.

#!/bin/bash
a=3
b=4
c=3
if (("$a" < "$b")); then
    echo "$a is less than $b."
else
    echo "$a is not less than $b."
fi
if (("$a" != "$c")); then
    echo "$a is not equal to $c."
else
    echo "$a is equal to $c."
fi


# 计算
echo "$a + $b = $(($a + $b))"


# Output
# 3 is less than 4.
# 3 is equal to 3.
# 3 + 4 = 7

if [ condition-is-true ]
then
  command A
else
  command B
fi

# 或
if [ condition-is-true ]; then
  command A
else
  command B
fi

#!/bin/bash
MY_SHELL="csh"
if [ "$MY_SHELL" = "bash" ]
then
  echo "You are using the bash shell."
else
  echo "You are not using the bash shell."
fi

if [ condition-is-true ]
then
  command A
elif [ condition-is-true ]
then
  command B
else
  command C
fi
# or
if [ condition-is-true ]; then
  command A
elif [ condition-is-true ]; then
  command B
else
  command C
fi

#!/bin/bash
MY_SHELL="csh"
if [ "$MY_SHELL" = "bash" ]; then
  echo "You are using the bash shell."
elif [ "$MY_SHELL" = "csh" ]; then
  echo "You are using csh."
else
  echo "You are not using the bash shell."
fi

#!/bin/bash

VAR1="variable"
VAR2="variable 2"
if [[ (VAR1 == "variable")  ]]; then
    echo "They are the same."
else
    echo "Not the same."
fi

# 使用 &&
[[ ($VAR1 == variable) && (
$VAR2 == "variable 2") ]] && echo "They are the same again."

#!/bin/bash

digit=4
if [[ $digit =~ [0-9] ]]; then
    echo "$digit is a digit"
else
    echo "$digit isn't a digit"
fi
letter="abc"
if [[ $letter =~ [0-9] ]]; then
    echo "$letter is a digit"
else
    echo "$letter isn't a digit"
fi

# Output
# 4 is a digit
# abc isn't a digit

for VARIABLE_NAME in ITEM_1 ITEM_N
do
  command A
done

#!/bin/bash
for COLOR in red green blue
do
  echo "COLOR: $COLOR"
done

# Output
# COLOR: red
# COLOR: green
# COLOR: blue

#!/bin/bash
COLORS="red green blue"
for COLOR in $COLORS
do
  echo "COLOR: $COLOR"
done

#!/bin/bash
IMGS=$(ls *jpg)
DATE=$(date +%F)
for IMG in $IMGS
do
  echo "Renaming ${IMG} to ${DATE}-${IMG}"
  mv ${IMG} ${DATE}-${IMG}
done

$ scriptname param1 param2 param3

#!/bin/bash
echo "'\$0' is $0"
echo "'\$1' is $1"
echo "'\$2' is $2"
echo "'\$3' is $3"

$ ./param.sh
'$0' is ./param.sh
'$1' is
'$2' is
'$3' is

#!/bin/bash
for PARAM in $@
do
  echo "Param is: $PARAM"
done

$ ./params.sh a b c d e f
Param is: a
Param is: b
Param is: c
Param is: d
Param is: e
Param is: f

read -rp "PROMPT" VARIABLE

#!/bin/bash
read -rp "Enter your programming languages: " PROGRAMMES
echo "Your programming languages are: "
for PROGRAMME in $PROGRAMMES; do
    echo "$PROGRAMME "
done

$ ./read.sh
Enter your programming languages: go py
Your programming languages are:
go
py

$ echo {0..3}
$ echo {a..d}

# output: 
# 0 1 2 3
# a b c d

#!/bin/bash
for i in {0..9}; 
do 
  touch file_"$i".txt; 
done

$ ls -al file_*
-rw-rw-r-- 1 razeen razeen 0 2月  14 09:54 file_0.txt
-rw-rw-r-- 1 razeen razeen 0 2月  14 09:54 file_1.txt
-rw-rw-r-- 1 razeen razeen 0 2月  14 09:54 file_2.txt
...

#!/bin/bash
i=1
while [ $i -le 5 ]; do
  echo $i
  ((i++))
done

$ ls ./no/exist
ls: cannot access './no/exist': No such file or directory
$ echo "$?"
2

#!/bin/bash

HOST="razeen.me"
ping -c 1 $HOST
RETURN_CODE=$?
if [ "$RETURN_CODE" -eq "0" ]; then
    echo "$HOST reachable."
else
    echo "$HOST unreachable."
fi

$ ./ex.sh
PING razeen.me (47.108.161.7) 56(84) bytes of data.
64 bytes from 47.108.161.7 (47.108.161.7): icmp_seq=1 ttl=50 time=38.5 ms

--- razeen.me ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 38.541/38.541/38.541/0.000 ms
razeen.me reachable.

exit 0
exit 1
exit 2
etc.

#!/bin/bash
HOST="razeen.me"
ping -c 1 $HOST  
if [ "$?" -ne "0" ]
then
  echo "$HOST unreachable."
  exit 1
fi
exit 0

$ ./ex2.sh && ls
....
2021-02-13-132m93.jpg   ex2.sh      file_1.txt  file_4.txt  

mkdir /tmp/bak && cp test.txt /tmp/bak

cp test.txt /tmp/bak/ || cp test.test.txt /tmp

#!/bin/bash
host="razeen.me"
ping -c 1 $host && echo "You can reach ${host}."

#!/bin/bash
host="google.com"
ping -c 1 $host || echo "You can't reach ${host}."

cp text.txt /tmp/bak/ ; cp test.txt /tmp
# 等同于
cp text.txt /tmp/bak/  
cp test.txt /tmp

$ echo "$(cd ~ && ls)"
$ echo "$(cd ~ | ls)"

function function-name(){}
# 或
function-name(){}

#!/bin/bash
function hello(){
  echo "Hello!"
}
# 正确
hello

# 错误
# hello()

#!/bin/bash
function hello(){
  echo "Hello!"
  now
  bye
}
function now(){
  echo "It's $(date +%r)"
}
function bye(){
  echo "Bye bye."
}
hello

# Output
# Hello!
# It's 09:29:44 PM
# Bye bye.

#!/bin/bash
# this won't work
function hello(){
  echo "Hello!"
  now
}
hello
function now(){
  echo "It's $(date +%r)"
}

$ ./hello2.sh
Hello!
./hello2.sh: line 5: now: command not found

#!/bin/bash
function fullname(){
  echo "$0"
  echo "My name is $1 $2"
}

fullname Razeen Cheng


# Output
# ./func.sh
# My name is Razeen Cheng

#!/bin/bash
function greeting(){
  for NAME in $@
  do
    echo "Hi $NAME."
  done
}

greeting Tom Jerry

#!/bin/bash
my_func() {
  GLOBAL_VAR=1
}
# 这时，变量还是空的
echo "Calling GLONAL_VAR before calling function my_func"
# echo $GLOBAL_VAR

# 声明后，就可以输出了
my_func
echo "Calling GLONAL_VAR after calling function my_func"
echo $GLOBAL_VAR

#!/bin/bash 
MY_VAR=1
my_func () {
  local MY_VAR=2
  echo "my_func: MY_VAR=$MY_VAR" 
}
echo "global: MY_VAR=$MY_VAR"
my_func

$ my_function
$ echo $?
0

#!/bin/bash

# 该函数用来创建一个备份文件
function backup_file () {
    local BACK # 声明局部变量
    if [[ -f $1 ]];then # 检查参数（是否是文件）
        BACK="/tmp/$(basename "$1").$(date +%F).$$"
        echo "Backing up $1 to $BACK"
        cp "$1" "$BACK"
    else
        # 文件不存在.
        return 1
    fi
}

# 调用函数
if [[ "$1" ]]; then
    backup_file "$1"
    # if [[ $? -eq 0 ]]; then
    if [[ $(backup_file "$1") -eq 0 ]]; then
        echo "Backup succeeded."
        exit 0
    else
        echo "Backup failed."
        # 备份失败，中断，并返回非0状态.
        exit 1
    fi
else
    backup_file /etc/hosts
    echo "/etc/hosts Backup succeeded."
    exit 0
fi

$ ls /tmp
$ ex1           
Backing up /etc/hosts to /tmp/hosts.2020-10-04.77124
Backup succeeded.
$ ls /tmp
hosts.2020-10-04.77124