Every directory in linux is a child of root (/) directory.
Filesystem Hierarchy Standard1 2 (FHS), maintained by the Linux Foundation:
| Name | Functionality |
|---|---|
| /bin | binaries (commands) |
| /sbin | system binaries (system commands) |
| /lib | shared libraries for commands (bin & sbin) |
| /boot | files needed to boot the system |
| /dev | device nodes |
| /sys | device info files |
| /etc | editable text configuration files (.conf) |
| /home | contain user home folders |
| /var | variable files, keeps changing, logs & cache |
| /opt | optional files |
| /tmp | temporary files, can’t persist between reboots |
| /proc | running process information in files |
| /media | removable drives like USB |
| /mnt | temporarily mounted filesystems |
The /usr directory is the most misunderstood. It is a secondary hierarchy, which means it can have all these directories listed above inside of it and store non-essential data in them. It can be used as a shared directory to store data shared by all users and can be called a “Universal System Resources”.
History: In UNIX, /usr was a users directory (/usr/alice), but in GNU/Linux the user-specific directories goes in /home (/home/bob).
Index Nodes: Every file in linux filesystem has a number associated with it called an inode number, it is assigned in a sequential order.
All the info about a file is stored in a database called the inode table.
Links: two files can be linked together such that changes in data of one is reflected back in the other.
Types:
# softlink
ln -s file1 file2_link
# hardlink
ln file1 file2_link
# use ls command to show link target
$ ls -l
# unlink
unlink file2_link
A Good Demo: https://medium.com/@eightlimbed/hard-and-soft-symbolic-links-15eac92fb793
/ root
~ home dir (often home/johndoe)
. current dir
.. parent dir
- previous dir
Physical disks are often partitioned logically in linux.
BIOS-MBR allows upto 4 patitions but extended partitions can be created to make more logical partitions.
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| /sda1 | /sda2 | | extended partition | |
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| /sda1 | /sda2 | | /sda3 | /sda4 | /sda5 | |
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/sda3, /sda4, and /sda5 are part of the extended partition so we have a total of 5 partitions this way.
UEFI-GPT allows upto 128 partitions.
Mounting: Partitions can be mounted on a directory (/tmp, /home, etc…), it means that the directory path is a mountpoint to access the disk and whatever we write to/read from the mountpoint, it will use the partition.
| Size | Partition | Mounted at |
|---|---|---|
| 1GB | /dev/sda1 | /boot |
| 4GB | /dev/sda2 | linux-swap |
| 10GB | /dev/sda3 | /home |
| 985GB | /dev/sda4 | / |
We can change mountpoints with mount and unmount commands.
$ mount /dev/sda1 /foobar
$ unmount /dev/sda1
$ unmount /foobar # same as above
Used for virtual memory aka paging. There is a partition called linux-swap that can be created for this.
Debian: uses linux-swap partition
Ubuntu, Windows: saves paging data in a single file (pagefile)
Fedora, Android: uses zram: Sets aside temporary space on RAM itself and compresses & stores data there instead of the disk
There is no hard & fast rule for deciding on the swap partition size. You can choose any size but do note that swap is not a replacement for RAM as its much slower.
Linux has an ext family of formats unique to it, ext2, ext3, and ext4. Hard disk partitions can be formatted in this format.
Just like NTFS on Windows.
du (show disk usage of a file/dir)
df (show free disk space info of partitions)
$ du foo
12 foo
$ du - h foo # human readable sizes
12K foo
LVM is a special one, it can create partitions across disks; we can extend and shrink partitions using space from other disks too.