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基础知识

背景知识

  • 变量

    NAME="John"
    echo $NAME
    echo "$NAME"
    echo "${NAME}!"
  • 字符串引号

    NAME="John"
    echo "Hi $NAME"  #=> Hi John
    echo 'Hi $NAME'  #=> Hi $NAME 单引号不做解析
  • 函数

    get_name() {
    echo "John"
    }
    echo "You are $(get_name)"
  • 条件执行

    git commit && git push
    git commit || echo "Commit failed"
  • 执行命令

    echo "I'm in $(pwd)"
    echo "I'm in `pwd`"
  • 条件判断

    if [ -z "$string" ]; then
    echo "String is empty"
    elif [ -n "$string" ]; then
    echo "String is not empty"
    fi
  • 花括号展开

    echo {A,B}.js  => A.js B.js
    {A,B}    展开就是 A B
    {A,B}.js    展开就是 A.js B.js
    {1..5}    展开就是 1 2 3 4 5
  • 注释

单行注释用#符号

多行注释用冒号空格单引号 : '任意换行'

字符运算

name="John"
echo ${name}
echo ${name/J/j}    #=> "john" (substitution)
echo ${name:0:2}    #=> "Jo" (slicing)
echo ${name::2}     #=> "Jo" (slicing)
echo ${name::-1}    #=> "Joh" (slicing)
echo ${food:-Cake}  #=> $food or "Cake"
length=2
echo ${name:0:length}  #=> "Jo"
STR="/path/to/foo.cpp"
echo ${STR%.cpp}    # /path/to/foo
echo ${STR%.cpp}.o  # /path/to/foo.o

echo ${STR##*.}     # cpp (extension)
echo ${STR##*/}     # foo.cpp (basepath)

echo ${STR#*/}      # path/to/foo.cpp
echo ${STR##*/}     # foo.cpp

echo ${STR/foo/bar} # /path/to/bar.cpp
STR="Hello world"
echo ${STR:6:5}   # "world"
echo ${STR:-5:5}  # "world"
SRC="/path/to/foo.cpp"
BASE=${SRC##*/}   #=> "foo.cpp" (basepath)
DIR=${SRC%$BASE}  #=> "/path/to/" (dirpath)
${FOO%suffix}    删除后缀Remove suffix
${FOO#prefix}    删除前缀Remove prefix
${FOO%%suffix}    Remove long suffix
${FOO##prefix}    Remove long prefix
${FOO/from/to}    Replace first match
${FOO//from/to}    Replace all
${FOO/%from/to}    Replace suffix
${FOO/#from/to}    Replace prefix

数字运算

$((a + 200))      # Add 200 to $a
$((RANDOM%=200))  # Random number 0..200

循环

  • 基础循环

    for i in /etc/rc.*; do
    echo $i
    done
  • 死循环

    while true; do
    ···
    done
  • 指定范围

    for i in {1..3}; do
      echo "Welcome $i"
    done
    Welcome 1
    Welcome 2
    Welcome 3
    

    也可以指定步长

    for i in {5..20..5}; do
      echo "Welcome $i"
    done
    Welcome 5
    Welcome 10
    Welcome 15
    Welcome 20
    

函数

定义函数

myfunc() {
    echo "hello $1"
}
# 与上面相同 (alternate syntax)
function myfunc() {
    echo "hello $1"
}
myfunc "John"

返回值

myfunc() {
    echo "abc"
}
result=$(myfunc)
echo result

参数

$#    参数个数
$*    所有参数
$@    所有参数, starting from first
$1    第一个参数First argument

条件判断

完整参考:http://man7.org/linux/man-pages/man1/test.1.html

运算判断

# 字符串
[ STRING = STRING ]    等于
[ STRING != STRING ]    不等于
[ -z STRING ]    空字符
[ -n STRING ]    非空字符

# 数字
[ NUM -eq NUM ]    等于
[ NUM -ne NUM ]    不等于
[ NUM -lt NUM ]    小于Less than
[ NUM -le NUM ]    小于等于Less than or equal
[ NUM -gt NUM ]    大于Greater than
[ NUM -ge NUM ]    大于等于Greater than or equal

# 其他
[[ STRING =~ STRING ]]    正则Regexp 需要两个中括号
(( NUM < NUM ))    Numeric conditions
[ -o noclobber ]    If OPTIONNAME is enabled
[ ! EXPR ]    Not
[ X ] && [ Y ]    And
[ X ] || [ Y ]    Or

文件判断

[ -e FILE ]    文件存在(包含目录和文件)Exists
[ -r FILE ]    文件可读Readable
[ -h FILE ]    文件时链接Symlink
[ -d FILE ]    目录存在Directory
[ -w FILE ]    文件可写Writable
[ -s FILE ]    文件大小 > 0 bytes
[ -f FILE ]    文件存在(只是文件)File
[ -x FILE ]    文件可执行Executable
[ FILE1 -nt FILE2 ]    1 is more recent than 2
[ FILE1 -ot FILE2 ]    2 is more recent than 1
[ FILE1 -ef FILE2 ]    Same files

数组

定义数组

Fruits=('Apple' 'Banana' 'Orange')
Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

获取数组信息

echo ${Fruits[0]}           # Element #0
echo ${Fruits[@]}           # All elements, space-separated
echo ${#Fruits[@]}          # Number of elements
echo ${#Fruits}             # String length of the 1st element
echo ${#Fruits[3]}          # String length of the Nth element
echo ${Fruits[@]:3:2}       # Range (from position 3, length 2)

遍历数组

for i in "${arrayName[@]}"; do
  echo $i
done

操作数组

Fruits=("${Fruits[@]}" "Watermelon")    # Push
Fruits+=('Watermelon')                  # Also Push
Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
unset Fruits[2]                         # Remove one item
Fruits=("${Fruits[@]}")                 # Duplicate
Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
lines=(`cat "logfile"`)                 # Read from file

其他

子命令

(cd somedir; echo "I'm now in $PWD")
pwd # still in first directory

case

里面的内容可以正则表达式:

case "$1" in
  start | up)
    vagrant up
    ;;

  *)
    echo "Usage: $0 {start|stop|ssh}"
    ;;
esac

重定向

python hello.py > output.txt   # stdout to (file)
python hello.py >> output.txt  # stdout to (file), append
python hello.py 2> error.log   # stderr to (file)
python hello.py 2>&1           # stderr to stdout
python hello.py 2>/dev/null    # stderr to (null)
python hello.py &>/dev/null    # stdout and stderr to (null)
python hello.py < foo.txt # 读取内容

任务场景

  • 生成随机数:

    echo $((RANDOM%=200))
  • 生成随机IP:

    echo $((RANDOM%256)).$((RANDOM%256)).$((RANDOM%256)).$((RANDOM%256))
  • 根据文件路径列表,返回第一个存在的文件

filearray=("./myfile" "./dir1/myfile" "./dir2/myfile")
for mfile in "${filearray[@]}"; do
    if [[ -e $mfile ]]
    then
        # do what you want
        break
    fi
done
  • 根据时间生成文件名
file=myfile_$(date +"%Y%m%d%H%M%S")
# => myfile_20190117121316
  • 通过读取管道的列表通过进程池执行的方式实现并发 --max-procs 参数 或着-P参数可以指定并发数
fofacli 'domain=baidu.com' | xargs -n 1 --max-procs=3 -I{} bash -c "echo {} && wget {}"
  • 判断字符串包含
A="helloworld"
B="low"
if [[ $A == *$B* ]]
then
    echo "包含"
else
    echo "不包含"
fi
  • 取文件名
echo $(basename "/a/b.txt")
b.txt

也可以

a="/a/b.txt"; echo ${a##*/}
b.txt
  • 分隔符取左右的数据
a=abc-123
echo ${a%-*}
abc 
echo ${a##*-}
123
  • 最简单的条件判断打印行
[[ "1" == "1" ]] && echo "yes" || echo "no"
yes
[[ "1" == "2" ]] && echo "yes" || echo "no"
no
  • 正则表达式匹配数字 不能直接用\d的方式,可以用[[:digit:]]或者[0-9]
[[ "v0.0.10" =~ ^v[[:digit:]]+\.[[:digit:]]+\.[[:digit:]]+$ ]] && echo "yes" || echo "no"
yes
  • 遍历文件名
for filename in ./*; do echo $filename; done

常见问题

  • -e -f -d的区别是什么? -e是判断文件是否存在,包含-f和-d;-f只判断普通文件存在,-d只判断目录存在

  • 条件语句中一个中括号[]与两个中括号[[]]的区别是什么? 区分在于是否进行了单词切分(word splitting),比如如下语句报错:

    x='a b'; [ -s $x ] || echo "0"
    bash: [: a: 需要二元表达式

    换一种方式:

    x='a b'; [[ -s $x ]] || echo "0"
    0
    ```bash
    如上等同于:
    ```bash
    x='a b'; [ -s$x” ] || echo "0"
    0
  • 删除前缀和长前缀有什么区别? 类似于贪婪模式和非贪婪模式:

    bash-4.4$ X="abc.123.com";echo ${X%.*}
    abc.123
    bash-4.4$ X="abc.123.com";echo ${X%%.*}
    abc
    同样
    bash-4.4$ X="abc.123.com";echo ${X##*.}
    com
    bash-4.4$ X="abc.123.com";echo ${X#*.}
    123.com
  • 如何给函数传递参数? 直接调用的时候给参数就行,函数中通过$1,$2..的方式获取参数。