Linux.Shell Programming Notes - Getting Started Concept

This note is based on the linuxShell programming entry to the proficient book study experiment and related finishing notes

pdf file download: http://download.csdn.net/detail/ruishenh/6586391

Chapter 1 Concepts

How to Run a Program

There are three ways to run a Linux program

1. Use a file with executable permissions to run the file directly.

2. Directly invoke the command interpreter to execute the program

3. Use the source to execute the file

Linux executable commands are divided into three types: built-in commands, shell functions, and External command.

1. The built-in command is the command of the shell program itself. These commands are integrated into the shell interpreter. For example (cd)

2. Shell functions are a series of program code written in shell language that can be referenced like other commands.

3. An external command is an executable program that is independent of the shell. For example, find, grep, echo.sh. When the command line shell executes an external command, it creates a copy process of the current shell to execute. During the execution process, there is a process of creation and demise. The execution of external commands is as follows.

1 Use the POSIX system fork function interface to create a copy of the command line shell process (child process).

2 In the running environment of the child process, find the location of the external command in the Linux file system. If the external command gives the full path, skip this step.

3 In the child process, replace the shell copy with a new program and execute (exec), at this time the parent process goes to sleep, waiting for the child process to complete.

After the execution of the child process, the parent process then reads the next command from the terminal.

Note

(1) The child process is exactly the same as the parent process at the beginning of the creation, but the child process cannot change the parameters of the parent process.

(2) Only the built-in commands can change the property of the command line shell (environment variable).

When using shell to execute shell scripts, subprocesses are not created, but executed directly in the parent process!

Linux shell variables

variables

Variables are defined in many programming languages, with definitions of scope of use associated with variables. The essence of a variable is a key-value pair (key=value)

The name of a shell variable begins with a letter or an underscore, followed by a apostrophe, number, or underscore of any length. Unlike many other programming languages, shell variable name characters do not have a length limit. The Linux Shell does not distinguish between types of variables. All values ​​are strings, and like variable names, values ​​do not have a character length limit. The magic is that bash also allows comparison operations and integer operations. The key reason is whether the string value in the variable is a number.

Variable types: local variables and global variables.

Local variables must be specified with the local display when declaring. Global variables do not need to be modified. (environment variable)

Echo output

$ echo 'abc'$ echo $JAVA_HOME 
 Export import variable
exportPATH=/usr/local/pig/pig- 0.12.0/bin: $PATH 
 The env function and the set function are different env functions that display environment variables, and the set function displays all local variables, including the user's environment variables. For example, when the user sets 笠Var=123 on the command line, the set function will display the var variable, while the Env function will not show 
 (var is now a local changer, not an environment variable). If you use the export var = 123 command, both the set and Env functions can display the var variable. 
 Language Types
 Computers cannot directly understand high-level languages. They can only understand machine language directly, so you must translate high-level languages ​​into machine-language computers to execute programs written in high-level languages. 
 Languages ​​are generally divided into compiled and interpreted 
 Compiled languages ​​have exe files such as those used. binary file. Or the .class file (jar file) of the java program
 The interpreted type has js, python, shell, etc. 
 Linuxshell Advantages 
 1. Concise 
 The kernel environment in which the Linux Shell is located makes any advanced operations possible. 
 2. Easy to develop Because it follows the philosophy of Unix, optimized to today, it is easy to develop under the accumulation of predecessors
 3. Easy to transplant
 Run on any unix/linux
 Add ## to the shell script to introduce the 

#!/bin/bash #!/bin/Sh#!/bin/rm#!/bin/more