# Linux Commands Part 1 - 4

Tags: linux-com-book

## Introductory Commands

* `#` before terminal means super-user
* `date` to print date and time
  * `Sat Jun 18 12:01:17 AM IST 2022`
* `cal` to print calendar
* `df` to print disk drive and usage
* `free` to print memory
* `pwd` print working directory
* `Absolute PathName` : begins with root directory and follows the tree branch by branch till the desired folder
* `Relative PathName` : starts from the working directory
* `cd` to change directory
  * `cd -` to revert to previous directory
  * `cd ~username` : to change directory to another user
* ls : to list directories

  * `ls directory_1 directory_2` : to print list or multiple directories

  ```java
  ➜  ~ ls programm software
  programm:
  azure-dev-hackathon  inbox-app            
  configs              learning-springboot  
  demo                 playground           

  software:
  jetbrains-toolbox  tor-browser_en-US
  ```

  * `ls -l` : for more details
  * `ls -lt` : to list sorted acc. date modifed
    * `—reverse` : to reverse the sort

  ![List of options of ls command](/files/yLVfRKj0oaIMdLxUWbwI)

Long Listing Details

![details of fields printing long list](/files/90ZGPqO3Ug96p5SBSQiX)

* `file filename` : to print a brief description of the file’s contents
* `less` : to view content of file

## File Structure

![Linux file structure part 1](/files/qVOcetrjq8176kjQ5uBc)

![Linux file structure part 2](/files/Bjvmx0zIVAKyxfDsy6c5)

![Linux file structure part 3](/files/WBaDmDu9vjUMTI0fOB0y)

![Linux file structure part 4](/files/sNrCv70DWF4ot03PwW52)

![Linux file structure part 5](/files/YGSR3TjL9ncWjCW7x1Hf)

## Symbolic Links | Soft Link | Symlink

* symbolic links are like aliases or another name for the file

```java
lrwxrwxrwx 1 root root
11 2018-08-11 07:34 libc.so.6 ***->*** libc-2.6.so
```

**Why they are useful ?**

* Imagine we have a file with name `foo` . Whenever we update it we, change its name to foo-date-time, but the programs/user referencing it wouldst know that the name has change. To tackle this problem, we create a symlink of file foo → foo-date-time and the programs/user user only reference the symlink.
* So even if we change the name of file foo, it wont affect the programs as they would be referencing the symlink.

## Manipulating Files and Directories

* to create new directories
  * `mkdir dir_name`
* to create multiple directories
  * `mkdir dir1 dir2 dir3`
* to copy file to another directory
  * `cp item directory`
  * if `cp item1 item2` then content of item2 is replace with item1
* use `-a` file while copying to copy the attributes as well

![options for cp command](/files/md9t1ntWPwCFwLoDrSp0)

* The `mv` command performs both file moving and file renaming, depending on how it is used
* to move item to another directory `mv item... directory`
* to delete item `rm item`

![options for rm command](/files/7qN9tXhXbUwGxPH35cY6)

## Hard Links vs Soft Links

### Hard Link

* Hard links are the original Unix way of creating links
* Every file has a single hard link that gives the file its name
* Limitations
  * A hard link cannot reference a file outside its own file system
  * A hard link may not reference a directory
* A hard link is indistinguishable from the file itself

```bash
// for hard link
-> ln file.txt link

-> ls
-rw-r--r--. 1 udayyadav udayyadav  12 Jun 20 23:06 file.txt
-rw-r--r--. 2 udayyadav udayyadav  12 Jun 20 23:06 link1
```

### Soft Link

* They work by creating a special type of file that contains a text pointer to the referenced file or directory
* They are like windows shortcuts
* If file is deleted, then link points to nothing, it is broken

```bash
// for soft link
-> ln -s file.txt link

-> ls
lrwxrwxrwx. 1 udayyadav udayyadav   9 Jun 21 07:59 link2 -> file2.txt
```

## WildCards

![different wildcards present](/files/z2eTZdMc0UD9M83xAMmW)

![matching with different characters](/files/px5f92tWLx2Ens52JZFo)

![wildcard example](/files/aawgquU4HR3KcOYU2sny)


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