The main contents of this section

1. How to Get Help Documents
2. Introduction to Linux File System
3. Directory operation

1. How to get help documents

In the actual working process, we often forget how to use commands, such as what parameters can be followed by ls command. At this time, we can use man command to see how to use commands, such as

//man command acquisition command help manual
xtwy@ubuntu:~$ man ls

You can use the keyboard's

To display the next or previous line of commands, you can also use

View the previous or next page (screen) commands, and

The space bar can also be used to display commands on the next screen. To view the exit command, just press the q key to exit, or the h key to display the list of less commands (the man command outputs the results through the less command)

2. Introduction to Linux File System

(1) Documents and catalogues

This section introduces the Linux file system from the user's point of view. Linux divides files into directories and ordinary files according to the form of files, as follows:

The name of a directory or file shall not exceed 255 characters in length. The name of a file or directory may consist of the following characters:

• Uppercase letters (A–Z)
• Lowercase letters (a–z)
• Numbers (0–9)
• Underscore ( _ )
• Period(.)
• Comma(,)
  File or directory names are case-sensitive and belong to different files or directories

(2) File extensions and invisible filenames

Unlike Window s, Linux files do not require file extensions. For example, if you write a source file in C language, you can name it complier.c, or other file names such as complier, complier.ccc. This is not recommended, because if you can write a file extension with Linux file extensions, which are commonly used at present, are shown in the following table:

File name with extension	The Meaning of Extensions
max.c	C language source file
max.o	Coded target code file
max	Executable file corresponding to max.c
memo.txt	text file
memo.pdf	pdf files, you must use xpdf or kpdf on the GUI interface to view
memo.ps	PostScript files must be viewed using ghostscript or kpdf on the GUI interface
memo.z	Compressed files can be decompressed by uncompress or gunzip
memo.gz	The compressed files can be decompressed by gunzip
memo.tar.gz or memo.tgz	The tar archive file compressed by gzip can be decompressed by gunzip
memo.bz2	The file compressed by bzip2 can be decompressed by bunzip2
memo.html	html file, view using firefox in GUI environment
memo.jpg, etc.	Image file, open with photo viewer in GUI environment

In the previous lecture, we saw that there are still a large number of hidden files in linux, which can be displayed by ls-a command. If you want to define hidden files, just start with the file name or directory.

(3) Absolute path and relative path

Absolute path and relative path are very important concepts in Linux. The following figure shows what absolute path is.

All starting with the root directory "/" are absolute paths, and the rest are relative paths.

//Absolute path access
xtwy@ubuntu:~/Public$ cd /home/
xtwy@ubuntu:/home$ ls
xtwy
//Relative path access
xtwy@ubuntu:/home$ cd xtwy/

3. Directory operation

(1) Create directory mkdir

For the convenience of demonstration, the following directory structure is used for demonstration:

1 Absolute Path Creation Method

//Create with absolute paths
root@ubuntu:/home# mkdir /home/max
root@ubuntu:/home# ls
max  xtwy
root@ubuntu:/home# 

2 Relative Path Creation

//Create using relative paths
root@ubuntu:/home# mkdir max/names
root@ubuntu:/home# mkdir max/temp
root@ubuntu:/home# mkdir max/literature
root@ubuntu:/home# cd max
root@ubuntu:/home/max# mkdir demo
root@ubuntu:/home/max# ls
demo  literature  names  temp

Sometimes you don't want to create hierarchical directories. At this point, you can add the parameter - p (parents) after mkdir to create parent-child directories together.

root@ubuntu:/home/max# mkdir -p literature/promo
root@ubuntu:/home/max# ls
demo  literature  names  temp
root@ubuntu:/home/max# cd literature/
root@ubuntu:/home/max/literature# ls
promo

(2) Change directory cd

The difference between working directory and home directory
The default directory after each login is the home directory, which remains unchanged during the session with the system. The home directory is represented by ~

xtwy@ubuntu:/root$ cd ~
xtwy@ubuntu:~$ pwd
/home/xtwy

The working directory is also called the current directory. The directory after the execution of the cd command is the working directory. It can be changed at will.

//Represents the current directory, that is, the working directory.
//... Represents the upper directory of the current directory
xtwy@ubuntu:~$ cd .
xtwy@ubuntu:~$ cd ..
xtwy@ubuntu:/home$ 

(3) Delete directory rmdir

rmdir, short for remove directory, is used to delete a directory. It deletes all files in the directory first, and then deletes the directory. However, when there are subdirectories in the directory, the command cannot be executed. It needs to use rm commands, such as

//Delete temp directory, first delete files under the directory
//Remove temp directory itself
root@ubuntu:/home/max# rmdir temp/
root@ubuntu:/home/max# rmdir literature/
rmdir: failed to remove `literature/': Directory not empty
root@ubuntu:/home/max# rm -r literature/
root@ubuntu:/home/max# ls
demo  names

Among them, r in rm-r refers to the recursive deletion of directories and files in directories, so it has a strong destructive power, and should be used cautiously.

(4) Mobile directory mv

//Move the directory demo to / home/xtwy / directory
root@ubuntu:/home/max# mv demo/ /home/xtwy/
root@ubuntu:/home/max# cd /home/xtwy/
root@ubuntu:/home/xtwy# ls
demo     Documents  examples.desktop  Pictures  Templates
Desktop  Downloads  Music             Public    Videos
root@ubuntu:/home/xtwy# rmdir demo
//The demo directory of the original directory no longer exists
root@ubuntu:/home/xtwy# cd /home/max/
root@ubuntu:/home/max# ls
names

(5) Copy directory cp

Moving directories with the mv command, sometimes you need to copy directories. The following is how to use them:

//First create a demo directory, using - p, the parent directory will be created if it does not exist
root@ubuntu:/home/max# mkdir -p literature/demo
//Because literature also includes subdirectories, the copy is unsuccessful at this time
root@ubuntu:/home/max# cp literature/ /home/xtwy/
cp: omitting directory `literature/'
//If subdirectories are included, the - r parameter is added to indicate recursive copy.
root@ubuntu:/home/max# cp -r literature/ /home/xtwy/
root@ubuntu:/home/max# cd /homt
bash: cd: /homt: No such file or directory
root@ubuntu:/home/max# cd /home/xtwy/
root@ubuntu:/home/xtwy# ls
Desktop    Downloads         literature  Pictures  Templates
Documents  examples.desktop  Music       Public    Videos
root@ubuntu:/home/xtwy# cd literature/
root@ubuntu:/home/xtwy/literature# ls
demo

4. File operation

(1) Create files

There are many ways to create files directly from the command line. The common ways are as follows:

//Redirect the output command to the file through the echo command
root@ubuntu:/home/xtwy# echo "hello linux" > hello.txt
root@ubuntu:/home/xtwy# ls
Desktop    Downloads         hello.txt   Music     Public     Videos
Documents  examples.desktop  literature  Pictures  Templates
//touch command, how the file does not exist, will create the file
root@ubuntu:/home/xtwy# touch hell1.txt
root@ubuntu:/home/xtwy# ls
Desktop    Downloads         hell1.txt  literature  Pictures  Templates
Documents  examples.desktop  hello.txt  Music       Public    Videos

(2) Displaying the contents of documents

The cate command displays the contents of the file. Its full name is catenate, which means to connect words one by one.

root@ubuntu:/home/xtwy# cat hello.txt 
hello linux

The cat command displays everything in the file at once, such as

root@ubuntu:/home/xtwy# cat /etc/profile
# /etc/profile: system-wide .profile file for the Bourne shell (sh(1))
# and Bourne compatible shells (bash(1), ksh(1), ash(1), ...).

if [ -d /etc/profile.d ]; then
  for i in /etc/profile.d/*.sh; do
    if [ -r $i ]; then
      . $i
    fi
  done
  unset i
  ......

Sometimes we want to be able to view the contents of a file on a split screen. At this time we can use less or more paging programs. Less and more are used in a similar way. The next screen information is displayed by a space key. The difference between them is that less displays END messages at the end of the file, while more returns directly to the shell terminal, for example:
less command

more command

(3) Copy files by cp command

root@ubuntu:/home/xtwy# ls
Desktop    Downloads         hell1.txt  literature  Pictures  Templates
Documents  examples.desktop  hello.txt  Music       Public    Videos
//Copy files
root@ubuntu:/home/xtwy# cp hell1.txt literature/demo
root@ubuntu:/home/xtwy# cd literature/demo
//cd - Return to the last working directory executed
root@ubuntu:/home/xtwy/literature/demo# cd -
/home/xtwy

It should be noted that when the cp command is copied, if the file already exists in the target directory, the system will not give a warning, but directly overwrite it. Therefore, it may have the risk of destroying the file. To solve this problem, we can use the - i parameter to let the system give a warning, for example:

root@ubuntu:/home/xtwy# cp -i hell1.txt literature/demo
cp: overwrite `literature/demo/hell1.txt'? 

(3) mv command to move or rename files

//In the same directory, it is equivalent to file renaming, and hell1.txt does not exist after execution.
root@ubuntu:/home/xtwy# mv hell1.txt hell2.txt
root@ubuntu:/home/xtwy# ls
Desktop    Downloads         hell2.txt  literature  Pictures  Templates
Documents  examples.desktop  hello.txt  Music       Public    Videos
//Move hell2.txt to literature/demo
root@ubuntu:/home/xtwy# mv hell2.txt literature/demo
root@ubuntu:/home/xtwy# cd literature/demo/
root@ubuntu:/home/xtwy/literature/demo# ls
hell1.txt  hell2.txt
root@ubuntu:/home/xtwy/literature/demo# cd -
/home/xtwy
//Source directory hell2.txt no longer exists
root@ubuntu:/home/xtwy# ls
Desktop    Downloads         hello.txt   Music     Public     Videos
Documents  examples.desktop  literature  Pictures  Templates

(4) Display the head or tail of a document

The header command is used to display the contents of the file header, tail command is used at the end, and the default number of lines is 10.

root@ubuntu:/home/xtwy# head ~/.bashrc 
# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
# for examples

# If not running interactively, don't do anything
[ -z "$PS1" ] && return

# don't put duplicate lines in the history. See bash(1) for more options
# ... or force ignoredups and ignorespace
HISTCONTROL=ignoredups:ignorespace
root@ubuntu:/home/xtwy# tail ~/.bashrc
if [ -f ~/.bash_aliases ]; then
    . ~/.bash_aliases
fi

# enable programmable completion features (you don't need to enable
# this, if it's already enabled in /etc/bash.bashrc and /etc/profile
# sources /etc/bash.bashrc).
#if [ -f /etc/bash_completion ] && ! shopt -oq posix; then
#    . /etc/bash_completion
#fi

The default number of rows for head and tail can be modified, for example:

//Show only the first two lines
root@ubuntu:/home/xtwy# head -2 ~/.bashrc 
# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)

Tail command may often be used when viewing the growth of log file content. For example, after hadoop is started, many logs will be generated, but when problems occur, tail command can be used to dynamically monitor the growth of log file content and see where the problem is.

//Initial display
root@ubuntu:/home/xtwy# tail -f hello.txt 
hello linux

//Adding content to a document
root@ubuntu:/home/xtwy# echo "hello linux linux" >> hello.txt

//Additional output
root@ubuntu:/home/xtwy# tail -f hello.txt 
hello linux
hello linux linux

(5) Other Common Document Operating Commands

Neither of the following commands will change the contents of the file

root@ubuntu:/home/xtwy# cp hello.txt hello1.txt
root@ubuntu:/home/xtwy# ls
Desktop    Downloads         hello1.txt  literature  Pictures  Templates
Documents  examples.desktop  hello.txt   Music       Public    Videos
//Sort by file content
root@ubuntu:/home/xtwy# sort hello1.txt
hello linux
hello linux linux
//Inverse Sequence Output
root@ubuntu:/home/xtwy# sort -r  hello1.txt
hello linux linux
hello linux
//diff for content comparison
root@ubuntu:/home/xtwy# diff hello1.txt hello.txt
//Adding content to a document
root@ubuntu:/home/xtwy# echo "hello linux linux" >> hello.txt
//Content comparison
root@ubuntu:/home/xtwy# diff hello1.txt hello.txt
2a3
> hello linux linux
//Format output
//- The u parameter divides the file into blocks
//The two files for comparison are represented by -, +, respectively.
//In this case - for Hello 1. txt, +for hello.txt
root@ubuntu:/home/xtwy# diff -u hello1.txt hello.txt
--- hello1.txt  2015-08-22 17:28:44.071202558 -0700
+++ hello.txt   2015-08-22 17:29:49.131181281 -0700
//@@ xxx @ is used to identify the start number and number of rows
//- 1,2 means hello 1.txt file start number is 1, line number is 2
//+ 1,3 means the hello.txt file has a starting number of 1 and a line number of 3.
@@ -1,2 +1,3 @@
 hello linux
 hello linux linux
+hello linux linux

Author: foreknow
Links: https://www.jianshu.com/p/d3c673b3f905
Source: Brief Book
The copyright of the brief book belongs to the author. For any form of reprinting, please contact the author for authorization and indicate the source.