有用的Shell片段整理

## 截取字符串的最后一位
``` bash
# echo $var
3234j35ja
# echo ${var: -1}  #注意：-1前要有个空格
a
```

## 来自网络
1. 很多很全很强大的http://tldp.org/LDP/abs/html/contributed-scripts.html

## 参数校验
### 校验参数个数
``` bash
#!/bin/sh

#-------------------------------------
#  this function is used to check the number of parameters
#-------------------------------------

E_WRONG_ARGS=85

# parameters
script_parameters="-a -h -m -z"

Number_of_expected_args=4

if [ $# -ne $Number_of_expected_args ]
then
        echo "Usage: `basename $0` $script_parameters"
        # `basename $0` is the script's filename
        exit $E_WRONG_ARGS
fi
```

### 特殊字符
``` bash
# {} 双括号用来表示多个

echo \"{These,words,are,quoted}\"   # " prefix and suffix
# "These" "words" "are" "quoted"

cat {file1,file2,file3} > combined_file
# Concatenates the files file1, file2, and file3 into combined_file.

cp file22.{txt,backup}
# Copies "file22.txt" to "file22.backup"

echo {a..z} # a b c d e f g h i j k l m n o p q r s t u v w x y z
# Echoes characters between a and z.

echo {0..3} # 0 1 2 3
# Echoes characters between 0 and 3.

base64_charset=( {A..Z} {a..z} {0..9} + / = )
# Initializing an array, using extended brace expansion.
# From vladz's "base64.sh" example script.

```

### echo
``` bash
hello="A B  C   D"
echo $hello   # A B C D
echo "$hello" # A B  C   D
# As we see, echo $hello   and   echo "$hello"   give different results.
echo '$hello' # $hello
```

### 批量命令

#### 批量创建文件夹
``` bash
mkdir rc{0,1,2,3,4,5,6,S}.d
```
上面这个比下面这个要简单
``` bash
for runlevel in 0 1 2 3 4 5 6 S
do
  mkdir rc${runlevel}.d
done
```

#### ls也可以批量
``` bash
$ cd /
$ ls -ld {,usr,usr/local}/{bin,sbin,lib}
drwxr-xr-x    2 root     root         4096 Oct 26 01:00 /bin
drwxr-xr-x    6 root     root         4096 Jan 16 17:09 /lib
drwxr-xr-x    2 root     root         4096 Oct 27 00:02 /sbin
drwxr-xr-x    2 root     root        40960 Jan 16 19:35 usr/bin
drwxr-xr-x   83 root     root        49152 Jan 16 17:23 usr/lib
drwxr-xr-x    2 root     root         4096 Jan 16 22:22 usr/local/bin
drwxr-xr-x    3 root     root         4096 Jan 16 19:17 usr/local/lib
drwxr-xr-x    2 root     root         4096 Dec 28 00:44 usr/local/sbin
drwxr-xr-x    2 root     root         8192 Dec 27 02:10 usr/sbin
```

### 判断变量是否存在
下面的命令，如果PID不存在，会报错
``` bash
test $PID -gt 0
```
结果：
``` bash
BJ1431:practice momo$ test $PID -gt 0
-bash: test: -gt: unary operator expected
```

即使先判断PID是否为空也不行：
``` bash
BJ1431:practice momo$ test -n $PID && test $PID -gt 0
-bash: test: -gt: unary operator expected
```

`-n`是判断字符串长度是否非0，但是如果变量不存在，也会返回true，所以正确的办法是用双引号把变量抱起来。
``` bash
test -n "$PID" && test $PID -gt 0
```
**所以是不是针对字符串的判断，最好都用双引号括起来呢？**

### 为变量定义默认值
#### 复杂的方式
``` bash
#!/bin/sh
echo -en "What is your name [ `whoami` ] "
read myname
if [ -z "$myname" ]; then
  myname=`whoami`
fi
echo "Your name is : $myname"
```

#### 更简单的方式
``` bash
echo -en "What is your name [ `whoami` ] "
read myname
echo "Your name is : ${myname:-John Doe}"
```
或
``` bash
echo "Your name is : ${myname:-John Doe}"
```
或
``` bash
echo "Your name is : ${myname:=John Doe}"
```

**通过`:-`或`:=`可以为shell中的变量赋默认值，变量需要用${}包围**

### awk 显示匹配行的上一行或下一行

假设有文件 out.log
``` bash
2017-05-07 19:25:43,982 INFO
        message received: hello how are you ?
2017-05-07 19:25:45,415 INFO
        message received: what is your name ?
```
上面这个文件比较奇怪，时间戳和info内容不在同一行，那么以下两个需求如何实现呢？
1) 查找`hello how are you ?`消息接收的时间
2）查找`2017-05-07 19:25:45,415`时间接收到的消息内容

当然用grep可以解决问题，但是我们要用awk来实现（awk在控制输出方面更强大）。我们需要做的就是输出匹配行的上一行或者下一行。

### 显示匹配行的上一行
``` bash
awk '/hello how are you/ {print a;print $0}{a=$0}' out.log
```
说明：
上面的执行过程用ifelse表示相当于
``` bash
while (not end){
    if (current_lines contains 'hello how are you'){
        print a;
        print $0;
    }
    a = current_line;
}
```
每次都把当前行存入变量a,当前行匹配的时候，输出变量a，此时a实际上是上一行的内容
注意：
`a=$0`是单独放在`{}`中的。

### 显示匹配行的下一行
``` bash
awk '/2017-05-07 19:25:45,415/{print;getline;print}' out.log
```
说明：
> getline 是awk的内置函数，用于读取下一行。
> print; 默认输出当前操作的行，相当于print $0.

## shell quick reference
``` bash

Command	     Description	  Example
&	Run the previous command in the background	ls &
&&	Logical AND	if [ "$foo" -ge "0" ] && [ "$foo" -le "9"]
||	Logical OR	if [ "$foo" -lt "0" ] || [ "$foo" -gt "9" ] (not in Bourne shell)
^	Start of line	grep "^foo"
$	End of line	grep "foo$"
=	String equality (cf. -eq)	if [ "$foo" = "bar" ]
!	Logical NOT	if [ "$foo" != "bar" ]
$$	PID of current shell	echo "my PID = $$"
$!	PID of last background command	ls & echo "PID of ls = $!"
$?	exit status of last command	ls ; echo "ls returned code $?"
$0	Name of current command (as called)	echo "I am $0"
$1	Name of current command's first parameter	echo "My first argument is $1"
$9	Name of current command's ninth parameter	echo "My ninth argument is $9"
$@	All of current command's parameters (preserving whitespace and quoting)	echo "My arguments are $@"
$*	All of current command's parameters (not preserving whitespace and quoting)	echo "My arguments are $*"
-eq	Numeric Equality	if [ "$foo" -eq "9" ]
-ne	Numeric Inquality	if [ "$foo" -ne "9" ]
-lt	Less Than	if [ "$foo" -lt "9" ]
-le	Less Than or Equal	if [ "$foo" -le "9" ]
-gt	Greater Than	if [ "$foo" -gt "9" ]
-ge	Greater Than or Equal	if [ "$foo" -ge "9" ]
-z	String is zero length	if [ -z "$foo" ]
-n	String is not zero length	if [ -n "$foo" ]
-nt	Newer Than	if [ "$file1" -nt "$file2" ]
-d	Is a Directory	if [ -d /bin ]
-f	Is a File	if [ -f /bin/ls ]
-r	Is a readable file	if [ -r /bin/ls ]
-w	Is a writable file	if [ -w /bin/ls ]
-x	Is an executable file	if [ -x /bin/ls ]
```

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