Shell Basics1

傷城~ 2023-07-14 05:42 16阅读 0赞

### 文章目录 ###

*  Getting Started
 *  Shell
 *   *  Shell usage
     *  Shell – Environment variable
 *  Variables
 *  Basic commands
 *   *  Man(ul) pages
     *  Directory Navigation导航
     *   *  Other usage of ls
         *  Interact with files and directories
 *  What’s in a file? \[again\]
 *  Input and Output (I/O)
 *   *  Output redirection
     *  Error redirection
     *   *  Summary (out & err redirection)
     *  Input redirection

# Getting Started #

*  Everything in Linux is considered to be either a file or a process:
 *  A **process** is an executing program identified by a unique process identifier, called a PID.
 *  A **file** is a collection of data, with a location in the file system called a path. (separated by forward slashes “/”)
 *  A **directory** is a special type of file, which is used to hold information about other files.
 *  A **symbolic link** is a special type of file which refer to another file in the filesystem. 软连接  
    [Linux中硬连接(hard link)与软连接(symbolic link)的区别][Linux_hard link_symbolic link]

# Shell #

*  There are many Shells.  
    sh (Bourne shell), csh (C shell), tcsh  
    ksh (Korn shell), pdksh (Public domain ksh)  
    bash (Bourne-agin shell)  
    zsh
 *  Which Shell am I running?  
    Various prompts are used in different shells

1.  bash usually have $ in the prompt
2.  C shell uses %
3.  tcsh users often use >
4.  Superuser \#

~$ echo $SHELL

~$ grep yourloginname /etc/passwd

~$ cat /etc/shells

## Shell usage ##

~$ <option(s)> <argument(s)>

~$ which grep

*  Tab completion

## Shell – Environment variable ##

~$ echo $PATH 
    ~$ export

# Variables #

*  Add `${var_name}` to use a variable (\{\} optinal)
 *  No`[space]` while defining a variable
 *  Environment variables: `$SHELL`, `$PATH`, …
 *  System variables: `$$`, `$?`, `$1`, …

# Basic commands #

*  Show what’s in a file:`~$ cat <filename>`
 *  Demonstrate the system time: `~$ date`
 *  List all of your own current running processes: `~$ ps`

## Man(ul) pages ##

`~$ man <program or command>`  
`~$ man –k <command>`

<table> 
 <thead> 
  <tr> 
   <th>Section</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td>Executable programs and shell commands.</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>System calls (functions provided by the kernel)</td> 
  </tr> 
  <tr> 
   <td>3</td> 
   <td>Library calls (functions within program libraries)</td> 
  </tr> 
  <tr> 
   <td>4</td> 
   <td>Special files</td> 
  </tr> 
  <tr> 
   <td>5</td> 
   <td>File formats and conventions</td> 
  </tr> 
  <tr> 
   <td>6</td> 
   <td>Games</td> 
  </tr> 
  <tr> 
   <td>7</td> 
   <td>Miscellaneous (including macro packages and conventions)</td> 
  </tr> 
  <tr> 
   <td>8</td> 
   <td>System administration commands (usually only for root)</td> 
  </tr> 
 </tbody> 
</table>

## Directory Navigation导航 ##

*  navigate command: `~$ cd <directory name>`
 *  **root** directory: `/`
 *  **home** directory: `~`
 *  current directory: `~$ pwd`
 *  `ls` command: display the files in the **current** directory or any directory specified with a path
 *  Use `–a` option to display all files (dot file included)
 *  Use `–l` option to display in long format (detailed info)  
    ![在这里插入图片描述][20200314140802972.png]  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM5NzUzNzc4_size_16_color_FFFFFF_t_70]

### Other usage of ls ###

*  `-t` option: Oldest Files
 *  `-u` option: Newest Files
 *  use the wildcard \* to view files of a specific type `~$ ls *.h`

### Interact with files and directories ###

`mkdir` – make a new directory of the given name  
`mv` – move files, directories, or both to a new location  
`~$ mv file1 Newdir`  
Tips: use mv to rename files;  
`~$ mv file1 file2`  
Tips: use wildcard like `*` to move all files of a specific type  
`~$ mv *.c ../CodeDir`  
`cp` – copy files, directories, or both to a new location  
Tips: to copy a directory, use `–r` option (recursively)  
`rm` – remove files or directories permanently永久的 from the system  
Tips: DO NOT TRY `sudo rm /*`  
`alias`: [设置指令的别名][Link 1]  
alias\[别名\]=\[指令名称\]

> Which of the follow behavior can somehow help to avoid using “rm /\*” accidentally?
> 
>  *  Take good care when using “rm”
>  *  Always use –i option with rm
>  *  Add alias rm=‘mv $1 ./.trash’

> I want to filter out a specified type of files (say like extension .zj) in “myfolder” and move  
> them to a new directory “mywork”. Which of these command do I need to do?
> 
>  *  mv \*.zj mywork/
>  *  cd myfolder
>  *  mkdir mywork

> Suppose I am a writer, and I’ve finished writing my new book with independent text files for each chapter, such as chap1.txt, chap2.txt, …, along with a “prologue”, “contents”, etc.  
> Which command would better help me to examine all the chapter txt file from the others?
> 
>  *  cat chap\*.txt

# What’s in a file? \[again\] #

`more`: compare with cat, show “more” detailed info of the file, use \[space\] to navigate  
`less`: “LESS is MORE”  
`head`: the command head shows the first 10 lines  
`tail`: the command tail –n shows the final n lines  
`~$ cat file`  
`~$ cat chap1 chap2 chap3`  
`~$ cat chap1 chap2 chap3 > book`  
`~$ cat phone_bill_this_month >> phone_bill_this_year`

# Input and Output (I/O) #

*  Inputs and outputs of a program are called **streams** in Linux.
 *  There are three types of streams:  
    `stdin (0)` – stream data going into a program (by default from the keyboard)  
    `stdout (1)`– data is written out by a program (by default to the screen)  
    `stderr (2)`– program output error messages (by default to the screen)
 *  Use “>” or “<” to redirect the streams

## Output redirection ##

*  With the redirection operator `>`, we can save the output (`stdout`) from a program to a file
 *  追加模式`>>`  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM5NzUzNzc4_size_16_color_FFFFFF_t_70 1]

## Error redirection ##

*  `stdout` and `stderr` both consider screen as the default output target.
 *  With redirection operator `2>`, we can redirect only stderr to a separate file  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM5NzUzNzc4_size_16_color_FFFFFF_t_70 2]

> Command “history” displays last 1000 history input prompts by default. Please try these following command in this order in your terminal and think what happened? and What’s in myhistory?  
> history > myhistory  
> history >> myhistory  
> history 2> myhistory  
> history 2>> myhistory

*  What happened?  
    `wc` command: count word in the file  
    with –l option: only displays the line count  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM5NzUzNzc4_size_16_color_FFFFFF_t_70 3]
 *  What’s the meaning?  
    Please try command “`strace`”  
    strace常用来跟踪进程执行时的系统调用和所接收的信号。 在Linux世界，进程不能直接访问硬件设备，当进程需要访问硬件设备(比如读取磁盘文件，接收网络数据等等)时，必须由用户态模式切换至内核态模式，通 过系统调用访问硬件设备。strace可以跟踪到一个进程产生的系统调用,包括参数，返回值，执行消耗的时间。  
    `~$ strace ls` it will display all the system call when execute an `ls` command
 *  what happens when execute `~$ strace ls >log1`?
 *  what happens when execute `~$ strace ls 2>log2`?

### Summary (out & err redirection) ###

*  difference between > and >>
 *  descriptors of stdin, stdout and stderr are 0, 1, 2 respectively
 *  thus, use > or 1> to redirect stdout, `2>` to redirect stderr  
    Tips: redirect out to multiple files  
    `~$ command|tee outputfile1 outputfile2`  
    `~$ command|tee –a outputfile`  
    redirect stderr with tee  
    `~$ command 2>&1|tee outputfile` 把错误流和输出流合并

## Input redirection ##

*  By using the redirection operator <, Input can also be given to a command from a file instead of typing it in the shell  
    `~$ cat > inputfile < source_file`  
    `<<`indicates that the input stream ends after typing the following \[symbol\]  
    `~$ cat > inputfile << EOF`

> I have two files in current directory: a and b, I want to copy data in b to a, which command would be useful?
> 
>  *  cp a b
>  *  cat > a < b

[Linux_hard link_symbolic link]: https://blog.csdn.net/gxzc936733992/article/details/49340429
[20200314140802972.png]: /images/20230529/b3c1817b75744ef7bbbb58fe699f171e.png
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[Link 1]: https://editor.csdn.net/md/?articleId=104984817
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM5NzUzNzc4_size_16_color_FFFFFF_t_70 1]: /images/20230529/8e85a01bc37e42678026bada0cbaa999.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM5NzUzNzc4_size_16_color_FFFFFF_t_70 2]: /images/20230529/d4b1cec91ebf4d84aa3afab44c7ec59d.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM5NzUzNzc4_size_16_color_FFFFFF_t_70 3]: /images/20230529/6c9d10f61131461992dd1635b5f5e302.png