Linux-Shell Basic Study

shell 脚本看起来是这样的

#！/bin/env bash

# def var
DATE=`date`
LOG_FILE=`date +%Y%m%d`.log

# use var
echo $DATE
echo "$DATE"
echo ${DATE}    # ${} 还有额外的用法

# 注意不加 "" 的引用和有 "" 是不一样的
HELLO="AB C    D"
echo $HELLO    # AB C D
echo "$HELLO"  # AB C    D

exit $?

• 脚本解释行 #! path
  用于表明解释该文本的解释器，类似的用法还有 #!/usr/bin/python #!/bin/lua

---

分支语句

if EXPRESSION 
then
    echo "do something"
fi

# or
if EXPRESSION; then
    echo "do something"
fi

# muti branches
if EXPRESSION
then
    echo "do something 1"
elif EXPRESSION
then
    echo "do something 2"
else
    echo "do something 3"
fi

# switch...case
case "$variable" in 
 "$condition1" ) 
 command... 
 ;; 
 "$condition2" ) 
 command... 
 ;; 
esac

# Example  眼熟？ABS 上搬过来的
case "$Keypress" in
  [[:lower:]]   ) echo "Lowercase letter";;
  [[:upper:]]   ) echo "Uppercase letter";;
  [0-9]         ) echo "Digit";;
  *             ) echo "Punctuation, whitespace, or other";;
esac 

循环语句

# 范围语句 for
for arg in [list]
do 
 command(s)... 
done

# 例子
for num in 1 2 3 4 5 6
do 
  echo "$num"
done 

for num in `seq 6`
do
  echo "$num"
done 

for num in {1..6}
do
  echo -n "$num "
done  

# C 风格
LIMIT=10
for ((a=1, b=1; a <= LIMIT ; ++a, ++b))
do  # The comma concatenates operations.
  echo -n "$a-$b "
done

# while 语句
while [ condition ]
do 
  command(s)... 
done

# Example 
var0=0
LIMIT=10
while [ "$var0" -lt "$LIMIT" ]
do
  echo -n "$var0 "
  var0=`expr $var0 + 1`
done

# break / continue
break n       # 跳出第 n 层循环
continue n    # 跳出第 n 层循环，并继续

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