Files and folders

Learn how to manage files and folders.

• Files and folders
  • Folders (directories)
    • Navigation
      • Hierarchy
    • Information
    • Manipulation
  • Files
    • Manipulation
    • Editing
    • Reading
    • Information
    • Finding
  • Archives
  • Permissions
    • File/Folder permissions
    • Special permissions
      • SetUID & SetGID
      • Sticky bit
      • Finding files with special permissions set
    • Numeric permissions
    • Change file permissions
    • File ownership
    • Umask setting
    • ACL (Access Control Lists)

Folders (directories)

Navigation

• pwd: print working directory
• cd <path>: change directory
  • Use ../ to navigate upwards / to parent directory
  • Use /<...> for an absolute path (relative to root folder)
  • Use ~/<...> for a path relative to the current user home

Always wrap path names with spaces in quotes. (e. g. "/users/pi/my folder/ex.txt")

Hierarchy

The file system hierarchy is not identical on all distributions but thanks to the Filesystem Hierarchy Standard (FHS), there is a typical structure found on almost all distributions.

Current version is FHS 3.0 since 2015.

/ is the root folder and represents the start of the hierarchy

• /bin: Essential user command binaries
  • e. g. cat, chmod, echo, etc.
• /boot: Static files of the bootloader
• /dev: Device files
  • /dev/null: “black hole”
  • /dev/zero: infinite stream of zeroes
  • /dev/(u)random: infinite stream of random bytes
• /etc: Editable text configuration / config files
• /home: User home directories
  • /home/tux: would be tux’s home folder
• /lib: Essential shared libraries and kernel modules
• /media: Mount point for removable media
• /mnt: Mount point for temporarily mounted filesystems
• /opt: Add-on application software packages
• /proc: Virtual filesystems documenting kernel and process status as text files
• /root: Home directory for the root user
• /run: Run-time variable data
• /sbin: System binaries
• /srv: Data for services provided by this system
• /sys: Information about devices, drivers and some kernel features
• /tmp: Temporary files; often cleared on reboot
• /usr: Read-only user data; (Multi-)User utilities and applications; “User”/”Unix System Resources”
  • /usr/bin: user commands
  • /usr/include: header files, included by programs
  • /usr/lib: libraries
  • /usr/local: programs which are independent of the distro
    • /usr/local/bin
• /var: Variable files
  • /var/cache: Application cache data
  • /var/log: Logs
  • /var/run: Link to /run
  • /var/tmp: Temporary files preserved between reboots

Information

• ls [folder]: list the items in the currect or specified folder
• ls -l [folder]: same, but with details
• ls -ld [folder]: get infos about the current or specified folder

(Image source: www.ics.uci.edu - check out file permissions to understand the permission bits)

Manipulation

• mkdir <path>: create a directory
• rmdir <path>: delete a directory

Files

In Linux, a lot of things are files / file handles. This includes devices, pipes, sockets and symlinks.

Manipulation

• touch <path>: create an empty file
• rm <path>: delete a file
• cp <path_old> <path_new>: copy a file/dir
• scp <username@hostname>:<path_source> <path_target>: copy file from remote to local
• mv <path_old> <path_new>: move/rename a file/dir

Editing

• nano [+<line>] <path>: edit (and create) a file with the nano editor
  • Ctrl+o ENTER Ctrl+x to save and exit
• vi <path>: edit (and create) a file with the vi(m) editor
  • I to enter insert mode
  • Esc to exit insert mode / enter command mode
  • :wq to save and exit
  • :q! to exit without save

Reading

• cat <path>: print a file to console
• less <path>: read file in scrollable viewer (scroll up and down)
• more <path>: read file in scrollable viewer (scroll only down)
• tail <path>: print the end of a file
  • -n <N>: print the last N lines
  • -f: follow/watch the file for changes and print them continuously
• b cat <path>: print an image to console with the butterfly launcher (if installed)

Information

• wc <file>: get word count
• wc -l <file>: get line count

Finding

• find <folder> <search_params>: Searches the file system
  • -empty: Only empty files
  • -name '*searchstring*': Search by name
  • -user USERNAME: Owned by user
  • -type X: Filter by type (f = regular file, l = link, d = directory, p = pipe, s = socket)
  • -exec CMD {} \;: Run command for every item found (\; marks end of -exec section, {} the path placeholder)
• grep searchstring /var/myfirstfile: Searches for the pattern searchstring from the contents of /var/myfirstfile.
  • -i: ignore case
  • -R: read all files under directories recursiveley and follow symbolic links
  • -n: print line number

Archives

• tar -czvf <filename>.tar.gz /var/myfirstdirectory: compress a directory into an archive
• tar -xzvf <filename>.tar.gz: extract the archive
• zip [options] <target.zip> <source-files>: zip files
  • -r: recursively zip directory
• unzip [options] <file>: unzip a file
  • -j: into current directory

Permissions

Every file/folder is owned by a user and a group.

File/Folder permissions

• Permission groups: user, group and others.
• Permission types: read, write and execute.

Example permissions: -rwxrwxrwx+

The first positions stands for file (-), folder (d) or link (l). The three rwx triplets stand for the three permission groups user, group and others.

If a + is present at the end, it means that an ACL (Access Control List) is active.

Special permissions

More on this topic: here (external link)

SetUID & SetGID

Files that have the setUID or setGID bits set will be executed with the permissions of the owner (user/group).

Files created in folders that have the setUID or setGID bits set will not belong to the creator but to the owner (user/group) of the parent folder.

In ls -l, this is displayed in place of the x bit of the user/group (first/second rwx triplet): s = with execute permission; S = without execute permission

Sticky bit

The sticky bit shall only be applied to folders. If it is set, only the owner of a file in the folder can delete or rename it.

In ls -l, this is displayed in place of the x bit of others (third rwx triplet): t = with execute permission; T = without execute permission

Finding files with special permissions set

• find / -perm /4000 2>/dev/null: find files with setUID set
• find / -perm /2000 2>/dev/null: find files with setGID set
• find / -perm /1000 2>/dev/null: find files with sticky bit set

Numeric permissions

Permissions can also be represented using the octal (base 8) system.

Each of the three wxr triplets (owner user, owner group, others) is represented as one octal integer. w stands for 4, x for 2 and r for 1.

• Example 1: rwxr-xr-x = 755
• Example 2: rwxr-x--- = 750

Special permissions are represented by one octal digit before the others.

Numeric values:

Group	(Special perms)			User			Group			Others
Perm	setUID	setGID	sticky bit	r	w	x	r	w	x	r	w	x
Value	4000	2000	1000	400	200	100	40	20	10	4	2	1

Change file permissions

File permissions can be changed via chmod <perm/num> <path> command.

• chmod +x <path>: add execute permission to all three groups
• chmod g+w <path>: add write permission to the owner group
• chmod o=r <path>: only allow others to read
• chmod o-wx <path>: remove write and execute permissions from others
• chmod -w <path>: make the file read-only
• chmod u+s <path>: set setUID bit
• chmod g+s <path>: set setGID bit
• chmod +t <path>: set sticky bit
• chmod NUM <path>: set numeric permissions

(no letter before the symbol or a stands for all of them)

File ownership

• chown <username> <path>: transfer user ownership
• chown <username>:<group> <path>: transfer user and group ownership
• chown :<group> <path>: transfer group ownership
• chgrp <group> <path>: transfer group ownership

Umask setting

The umask setting defines, what permissions are removed by default on file/folder creation. Special permissions cannot be changed by the umask setting.

• File permissions = 0777 - umask
• Folder permissions = 0666 - umask

The umask can be set via umask command:

• umask <num>

ACL (Access Control Lists)

ACLs can be used to allow more users to access a file or folder.

• getfacl -a -e <file/folder>
• setfacl -m PERMS <file/folder>

PERMS is a list of entries separated by a comma. Every entry consists of type, name and permissions. If no type is specified, u for user is assumed.

• [u:]USERNAME:PERMISSIONS,g:GROUPNAME:PERMISSIONS,...
• u:uStudent3:rwx,g:gClass2:rx
• g:gClass2:r-x
• tux:-

ACLs are applied from top to bottom. The first matching rule will be applied!