shell 程序设计 | shell 基础

Aug 24 2018 Linux shell 4 minutes read (About 581 words)

shell 使用解释性语言.

Variables

One big difference between Shellscript and other programming languages is that there must be no spaces between “=” sign. It has its reason. Because when in a shell, if you use VAR = value, it’s just like running some command called “VAR” and two arguments named “=” and “value”. Therefore that is impossible to run successfully.

Shellscript uses $VAR to indicate it’s a variable that has been defined before.

Scope of variables

“local” variable

When we run a scipt, Linux creates a new process with a unique ‘pid’ for it. So we can only use variables created in this files in the process. Once the process is ending or we are in another process, we cannot use the variables.

“global” variable

However, like the concept of ‘environmental variables’ in Windows, Linux also has global-scope variables by using the source commands or . ./*.sh for sourcing a scipt file. This is how the configuration files like .bashrc or .vimrc work (source .bashrc everytime before opening a bash and source .vimrc everytime before openning a vim program).

the priority

Think a occasionally case, if a local defined variable has the same name as a global variable, the local one has priority. If the local one doesn’t exist or hasn’t been defined yet, use the global ones. If none of them exist, give out a null.

Statements

if语句

if [ 布尔表达式 ]
then
	若干语句
else
	若干语句
fi

case 语句

case 变量 in
    模式1 ) 若干语句;;
    模式2 ) 若干语句;;
esac

循环语句

循环语句有多种写法

for i in {1..10}
do
    若干语句
done

特殊符号

$$ 表示运行脚本的进程号(PID)

The use of curly braces “{}” can enclose a variable into it even if characters follow with no spaces. See the example below.

#!/bin/sh
echo "What is your name?"
read USER_NAME
echo "Hello $USER_NAME"
echo "I will create you a file called ${USER_NAME}_file"
touch "${USER_NAME}_file"

There is one more advantage that if we type two seperated words, the curly braces will prevent touch creating two new files for it.

Comment

Shellscript uses # to comment a whole line.

Running the script

Write scripts into a readable and executable file. The first line can indicate the program(/bin/sh) to run it. If you are using GNU/Linux, /bin/sh is normally a symbolic link to bash.

#!/bin/sh

Functions

buitin commands

echo: takes one or more parameters seperated by blank characters(spaces or tabs).

expr: Print the value of EXPRESSION to standard output. It supports logical expressions. It can be also used as a simple caculator for integers in shell. But caution that the asteroid * is regarded as a wildcard. So when you do multiplication, you should use \*.

read: reads a line from standard input into the variable supplied.

touch: create a empty file by the name

test: 可以用来检测某些文件或者是相关的属性，和判断符号 [] 等价
test -n 字符串: 字符串长度不为零则为真

For more Linux builtin commands, go to this article.

#Linux #shell