Filesystem and Permissions

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LinuxPage 1

Filesystem Hierarchy and Paths

1. One Tree, Rooted at /

Everything hangs off one tree rooted at /
No drive letters: disks and devices are mounted into the tree

Dir	Holds
`/etc`	config files
`/var`	logs, changing data
`/home`	user files
`/usr`	installed programs
`/tmp`	scratch space
`/dev`	devices
`/proc`	kernel and process info

2. Paths and Shortcuts

Absolute starts with /: /var/log/syslog
Relative starts from the current directory: ../logs

Symbol	Means
`.`	current dir
`..`	parent dir
`~`	your home
`-`	previous dir (`cd -`)

3. Common Mistakes

Paths are case-sensitive: Docs is not docs
A leading dot hides a file: .bashrc

Spaces

Spaces need quotes or a backslash: cd "My Docs" or cd My\ Docs

LinuxPage 2

Everyday File Commands: ls, cd, cp, mv, rm, find

1. Look Around

ls -lah: long format, all files incl. hidden, human-readable sizes
pwd prints where you are

2. Copy, Move, Delete

Command	Does	Note
`cp -r src/ dest/`	copies directories	needs -r
`cp -a`	also keeps permissions, timestamps	archive copy
`mv old.txt new.txt`	moves and renames	never needs -r
`rm -r dir`	deletes recursively	no trash, no undo

ls /path/check the target
rm -r /path/no undo

Danger line

rm -rf /path/ is the one to fear. Run ls on the path first.

3. find

find matches by name, age and type
Unlike locate, it walks the live disk
Add -exec rm {} \; to act on each result

bash

find /var -name "*.log" -mtime -1 -type f
find /var -name "*.log" -mtime -1 -type f -exec rm {} \;

-mtime -1 = changed within the last day

LinuxPage 3

Permissions: rwx and Octal

1. Reading a Mode

-rwxr-xr-- = type, owner, group, others
r=4, w=2, x=1; rwx=7, r-x=5, rw-=6, r--=4
640 = owner rw, group r, others none

Bit	File	Directory
`r`	read contents	list names
`w`	modify contents	create or delete entries
`x`	run it	cd into it

2. chmod

Octal sets all bits; symbolic changes only the bits named
Symbolic: who u g o a, op + - =, bits rwx

bash

chmod 755 script.sh
chmod u+x file
chmod g-w file
chmod o=r file
chmod a+r file

octal first, then symbolic

3. Mistakes

chmod 777 opens the file to everyone; give the exact bit needed
chmod -R 755 gives plain files x too: the mode is applied as given

Capital X

chmod -R u=rwX,go=rX dir adds x to directories only.

LinuxPage 4

Users, Groups, chown and sudo

1. Owners and accounts

Each file has one owner user and one owner group
id and whoami show who you are
Accounts: /etc/passwd; groups: /etc/group

2. chown, chgrp, groups

Command	Effect	Who
`chown alice:devs f`	owner + group	root only
`chown -R`	recurses	root only
`chgrp devs f`	group only	owner, if member

bash

sudo usermod -aG docker alice

without -a, other groups are dropped; log in again

3. sudo

sudo cmd runs as root with your password
Only if you are listed in /etc/sudoers
sudo -i opens a root shell
Edit the list only with visudo

4. Mistakes

sudo on everything leaves root-owned files in ~
Fix ownership with chown, don't run as root

Redirect runs as you

sudo echo x > f fails. Use echo x | sudo tee f

LinuxPage 5

Links: Hard vs Symbolic

1. Names and Inodes

Each file name points to an inode
Inode = metadata + pointers to the data blocks
A link is another name or pointer to it

2. Hard vs Symbolic

	Hard link	Symlink
Make	`ln target link`	`ln -s target link`
Is	2nd name, same inode	small file holding a path
Target deleted	data survives to last name	dangling
Cross fs / dirs	no	yes
`ls -l`	plain `-`	`l` and `->`

bash

ln a.txt hard.txt
ln -s a.txt soft.txt
ls -li

-i shows the inode number

3. Common Mistakes

ln -s order: target first, then link name
Relative target resolves from the link's folder, not your cwd

LinuxPage 6

Cheat Sheet: Files, Permissions, Users and Links

1. Navigate and Manage

pwd, cd -, ls -lah, tree -L 2
Paths: / absolute, ../ parent, ~ home

bash

cp -a src dst   # keeps perms, times
mv old new      # move or rename
rm -r dir       # no undo
mkdir -p a/b/c
touch f
find . -name "*.py" -type f

2. Octal Modes

Octal	Bits
7	`rwx`
6	`rw-`
5	`r-x`
4	`r--`
0	`---`

644 files, 755 scripts and dirs, 600 SSH keys, 700 private dirs

3. Access, Users and Links

chmod u+x f; chown user:group f (new owner needs root)
usermod -aG group user; id lists your groups
sudo -i root shell; visudo edits sudoers
Hard link: second name, same inode. Symlink: a path pointer

bash

ln target hard
ln -s target link
ls -li            # inode numbers
readlink -f link  # resolve symlink

Mistakes

chmod -R 755 makes plain files executable: use X. usermod without -a drops other groups.

LinuxPage 7

Check yourself

A file is -rw-r----- alice devs. Bob is in devs. Can Bob read it? Write to it?

Read: yes, group bits are r--.
Write: no, the group has no w.

sudo echo x > /etc/motd says permission denied. Why, and what fixes it?

The redirect > is done by your shell, not by sudo.
Fix: echo x | sudo tee /etc/motd.

After these commands, what do the two cat lines show?

cat h prints the contents: the inode lives on while a name remains.
cat s fails with no such file: the symlink is dangling.

ln data.txt h
ln -s data.txt s
rm data.txt
cat h; cat s