Linux Bash 总结
1. Bash 概念
主要有 4 种 shell 语言，Bash 是其中一种。四种 shell 分别是：
- bash (Bourne Again SHell)
- csh (C SHell)
- ksh (KornSHell)
- zsh
2. Bash 变量
- 定义变量
VAR="Hello World" VAR=123
注意："=" 前后不要有空格
- 使用变量
$VAR
${VAR}
- 设置只读变量
readonly VAR
- 取消一个变量
unset VAR
双引号，单引号区别
'' - 单引号中的字符不会做任何转换
"" - 双引号中的特殊字符会当做特殊字符处理，比如 "$" and "\" 会被识别成特殊字符。
`` - 执行 `` 中的命令
sudo yum install -y $(cat rpm_requirements.txt)
3. 字符串
A string can be defined with "", '' or nothing.
Get the length of a string
string="abcd"
echo ${#string} # output is 4
Cut out a string
echo ${var:0:5} # start from 0, length is 5, var[0,5)
Substitute
${VAR/a/A} # Substitute the first 'a' with 'A'
${VAR//a/A} # Substitute all 'a' with 'A'
${VAR//[ |:]/-} # Substitute all ' ' or ':' with '-'
4. 数组
Define an array
array_name=(value0 value1 value2 value3)
array_name=(
value0
value1
value2
value3
)
array_name[0]=value0
array_name[1]=value1
array_name[n]=valuen
Get an element of an array
${array_name[n]}
Get all elements of an array
${array_name[@]}
Get length of an array
${#array_name[@]}
${#array_name[*]}
Get length of element n
${#array_name[n]}
Through an array
for data in ${array[@]}; do
echo ${data}
done
5. 函数
Define firstly and then use it.
Two ways to define a function:
foo() {
}
function foo {
}
function foo() {
}
Use a function
foo
Use a function with parameter
var="Hello world"
foo ${var} # There will be two parameters, $1="Hello" $2="world"
foo "Hello world" # There will be one parameter, $1="Hello world"
Return value is returned by using "return xxx". xxx is in [0,255], saved in $?.
Return a string is invalid.
If no "return xxx" in function, return the result of last command.
Two ways to get the return value:
foo
i=$?
foo() {
echo 3
}
i=`foo`
Use keyword "local" to define a local variable in function.
Otherwise, the varibale in function is global.
Use keyword "exit" in function will exit the script.
6. Bash 的参数处理
$0 - bash name
$1 - first parameter
$2 - second parameter
$@ - all parameters
$* - all paramters
$# - number of paramters
shift
shift will shift parameter to left,
"shift" default "shift 1"
7. Bash 中的 if
## FORMAT
if [ condition ]; then
cmd
elif [ condition ]; then
cmd
else
cmd
fi
## EXAMPLE
if [ -e file1 ] # file1 exists
if [ -f file1 ] # file1 exists and is a regular file
if [ -s file1 ] # file1 exists and size > 0
if [ -L file1 ] # file1 exists and is a link
if [ -r file1 ] # file1 exists and is readable
if [ -w file1 ] # file1 exists and is writable
if [ -x file1 ] # file1 exists and is executable
if [ -d dir1 ] # dir1 exist
if [ file1 -nt file2 ] # file1 new than file2
if [ file1 -ot file2 ] # file1 old than file2
if [ -z string ] # string length is 0
if [ -n string ] # string length is not 0
if [ "$str1"x = "$str2"x ] # str1 == str2
if [ "$str1"x == "$str2"x ] # str1 == str2
if [ "$str1"x != "$str2"x ] # str1 != str2
if [ $num1 -eq $num2 ] # num1 == num2
if [ $num1 -nq $num2 ] # num1 != num2
if [ $num1 -lt $num2 ] # num1 < num2
if [ $num1 -le $num2 ] # num1 <= num2
if [ $num1 -gt $num2 ] # num1 > num2
if [ $num1 -ge $num2 ] # num1 >= num2
Note:
1. use []
2. after '[' and before ']' have a space
if [ -e file1 -a -e file2 ]
if [ -e file1 -o -e file2 ]
if [ ! -e file1 ]
if [[ exp1 && exp2 ]]
if [[ exp1 || exp2 ]]
8. Bash 中的 for
for ((i=0; i<10; i++)); do
echo $i
done
for ((i=0; i<10; i=i+2)); do
echo $i
done
for i in {1..5}; do
echo $i
done
for i in {1..100..2}; do
echo $i
done
for i in $(ls); do
echo $i
done
9. Bash 中的 while
while [ condition ]; do
cmd
done
10. Bash 中的 case
case $Command in
gzip)
tar czfP /backup/backupfile-`date +%F-%H-%M-%S`.tar.gz /etc/
;;
bzip2)
tar cjfP /backup/backupfile-`date +%F-%H-%M-%S`.tar.bz2 /etc/
;;
xz)
tar cJfP /backup/backupfile-`date +%F-%H-%M-%S`.tar.xz /etc/
;;
*)
echo "Usage:`basename $0`{gzip | bzip2 | xz }."
;;
esac
11. IO 重定向
ls > log
ls >> log
ls 2>&1 > log
wc -l < file
wc -l <delimiter
first line
second line
delimiter
12. Bash 中的数学计算
For int calculation:
val=`expr 2 + 2`
val=`expr $a + $b`
val=`expr $a - $b`
val=`expr $a \* $b`
val=`expr $b / $a`
val=`expr $b % $a`
Note: There should be space before and after operator.
13 Utils
Get current shell directory
SHELL_FOLDER=$(dirname $(readlink -f "$0"))
$? stores the return code.
