5.9 Files in Linux

✅ Linux

• file system: ext2, 3, 4

✅ Rules of files in linux

1️⃣ Files do not have extensions

• unless, you want to add an extension on purpose
• if you add an extension
• it becomes part of the name
• patata.txt
• name: patata, extension: txt ❌
• this is not a text file named patata ❌
• name: patata.txt
• this is a file that has name patata.txt without extension

2️⃣ Metadata of the file are

✔️ What is metadata?

• size of the file
• owner of the file
• date and time of creation
• date and time of last modified
• permissions: read/write/execute 👀 sohee can only read
  • what the person can do
• access control: who can do things with that file, users 👀 root, sohee
  • the users
• location: where the file is
• timestamps: modifications of the file, modification dates
• pointer to the file

✔️ Inodes table

• Where is the metadata?
• all the metadata is separated from the file itself
• inside the inodes table
• inodes table: saves metadata of the file
• inodes table is inside the OS in RAM
• 🆚 whereas the file iteslf will be stored in Harddisk

✔️ inside the Inodes table

• inside the table, there is a identifier inode number

✔️ How to read the file

• find the inode number
• go to the OS in RAM
• find the metadata of the file
• go to the harddisk and open the file

✔️ SWAP in Linux

• everytime something is modified in the file
• the OS in RAM is disturbed
• so the RAM is collapsed, exhausted
• 💊 create SWAP partition
• that is why in linux, create the SWAP partition
• nowadays, if you forget to add the SWAP partition for Linux
• linux will automatically create a file called SWAP file in the kernel
• SWAP file is dynamic, changes according to your needs
• 👎🏻 so maybe when you need it, there is no space
• 💊 create the SWAP partition for linux
• 🆚 in windows, metadata is a header next to the file
• that is why in windows, SWAp is not necessary

✅ Tree in linux

• cd: for entering a branch

✔️ How to name a directory/branch

• 1️⃣ absolute path
• getting to that directory starting by the root of the tree
• so it always starts with /

• bc you start with the root

• 👀 to go to x11 in absolute path

• /usr/X11R6/lib/x11

• 🛠️ if you want to reach directories close to root, use absolute

• 2️⃣ relative path
• getting to that directory starting from where I am
• recommended to start relative path with ./
• ./ means where I am now

• but ./ is optional

• 👀 If I am in usr
• ./X11R6/lib/x11

• X11R6/lib/x11 is also possible

• 🛠️ if you want to reach directories close to yourself, use relative
• 🛠️ use the path that is better to your performance

✔️ Four standard symbols in linux

• ./: where I am now
• ..: reach my parent branch, the branch I come from, one step down in the tree
  • 👀 If I am in x11, and I want to go to usr, do ../../.. or ./../../..
• ~: /home/personal
• /: root

✔️ We can create a hard link

• hard link
• @ = /mnt/dvd
• when we associate a path to a symbol
• hardlink: a different name for the same thing
• an extra name of smth
• we can create hardlinks for anything

Cesar is called cesar and borja
create a hard link to cesar ➡️ borja

• 👀 /: hardlink for the root
• 👀 ~: hardlink for /home/shpl
• 👀 .: hardlink for all directories I have visited

• 👀 ..: hardlink for all the directories that have children

• 🆚 hard links only exist in linux
• so in linux, one file can have 20 names!

- There is a file with two names
- you move one of those names to a hidden place
- and somebody deletes the other name,
- did the file disappear?
- No, you can still access it from the hidden name

- 👍🏻 when we create a hardlink
- we are protecting the file
- bc even if somebody deletes the file
- I can access it with the other name

✔️ How to delete a file forever in Linux

• The only way of deleting the file forever in linux
• is to delete all hardlinks

❓ When can you recover the deleted file?

- when you had a hardlink to that file
- 👍🏻 so you can recover deleted files
- by finding hardlinks in linux

✔️ One file has one inode number

• all the hardlink of a file
• have the same inode number

• as the file is the same

• one file can have several hardlinks
• one file can have one inode number ❓❓❓❓❓❓❓❓❓❓❓❓❓❓❓

❓ How to recover files in Linux?
- find pieces of the file in harddisk with inode number
- get the hardlink of the file
- then with the hardlink access the file

❓ How to recover files in harddisk?
- finding in the harddisk with same inode number

• create wierd hardlinks
• and moving them to other directories
• protect the files
• 👀 put passwords in music directory

❓ Two ways of protecting a branch in Linux

1. visit the file
- bc then the file will have hardlink . and the filename
- that is why linux visits all the branches when it boots

2. create sub-branch inside the branch
- that branch becomes parent
- so will also have hardlink ..
- enter the directory and create empty files inside

✔️ When the branch is deleted, hardlinks stay

• If you remove the branch lib
• but the branch lib had sub directories x11
• but to get to x11, you still have to go through ..
• even if you delete parent directory,
• the hard link of sub directory will not change!

✔️ In Linux, directory is the same as file

• hardlinks can be created to directories and files
• directory is a file with a type directory
• directory means container of other things

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