Modify Metadata

Rename

mv [old_name] [new_name]	Rename
rename 's/ /_/g' *	Replace spaces with underscores for all files.

Timestamps

touch	Updates access and modification to current datetime.
touch -t YYYYMMDDhhmm [file]	-t lets you customize the datetime. Without it, it sets to the current datetime.
touch -a -t YYYYMMDDhhmm [file]	Updates only access to specified datetime.
touch -m -t YYYYMMDDhhmm [file]	Updates only modification to specified datetime.
touch -d -t YYYYMMDDhhmm [file]	Updates access and modification to specific datetime.
touch -r <reference_file> [file]	Updates access and modification to match another file.

Updates *access and *modification of files found.

You can customize the find criteria to modify timestamps of specific files:

find -exec touch {} \;

Change the system time, which indirectly affects newly created or modified files:

date -s YYYYMMDDhhmm

This modifies timestamps of files listed by ls, extracting filenames with awk and using xargs to pipe them to touch:

ls -ltr | awk '{print $9}' | xargs -I {} touch -t YYYYMMDDhhmm {}

Note:

ctime (change time) of a file is updated to the current system time whenever the file's metadata or content changes.

It cannot be modified directly by any command.

mtime (modification time) is updated when the file's content changes

atime (access time) is updated when the file's content is accessed.

Ownership

chown [owner] [file]	Change owner.
chown [UID] [dir]	Change owner to UID.
chown [owner][:group] [file]	Changes owner and/or group of a file.
chown -R [owner][:group] [dir]	Recursively changes owner and/or group of a directory, sub-directories, and contents.
chgrp [group] [file]	Changes group of a file to the specified group name.
chgrp 100 directory1	Change group to GID.
chgrp -R [group] [dir]	Recursively changes group of a directory, sub-directories, and contents.

Permissions

chmod [mode] [file]	Changes permissions. (u+x, g-w, o=r) or (644, 755).
chmod +x [file]	Adds execute permission for the owner, group, and others. Shorthand for a+x or ugo+x.
chmod u=rwx,g=rx,o=r [file]	Sets the permissions explicitly.
chmod 755 [file]	Octal notation.
chmod -R mode [dir]	Recursively changes the permissions of a directory and all its contents.
chmod --reference=<ref_file> [file]	Match permissions of reference file.
umask	umask = "user file-creation mode mask". Sets the default creation permissions for new files and dirs.
umask 022	New default permissions on created files and dirs is 755.
chmod 4760 [file]	Add SUID bit
chmod 2760 [file]	Add SGID bit
chmod 1760 [file]	Add Sticky bit

Note:

4 = read (r)

2 = write (w)

1 = execute (x)

+ (add permission)

- (remove permission)

= (set permission exactly)

Attributes

chattr [+,-,=][attributes] [file]	Modify attributes
chattr =i [file]	Make file immutable, clears other attributes.

a (Append only): Can be added to, but existing data cannot be modified or deleted. This is useful for log files.

i (Immutable): Cannot be modified: it cannot be deleted, renamed, or linked, and no data can be written to it. Only the root user can set or clear this attribute.

s (Secure deletion): When a file with this attribute is deleted, its data is immediately overwritten with zeros. This makes recovering the file's data more difficult.

S (Synchronous update): This attribute ensures that modifications to the file are written synchronously to the disk, similar to how the sync command works.

A (No atime updates): Normally, the access time of a file is updated whenever it's read. Setting this attribute stops those updates, which can improve performance for certain applications.

Capabilities

setcap [capabilities] [file]	Set capabilities
setcap 'cap_net_bind_service+ep' [file]	Set capability as e-effective and p-permitted

Operators and Sets

+ (Add)

- (Remove)

= (Set): Explicitly sets capabilities, replacing any existing capabilities.

Effective (e): Which capabilities are "active" and can be used by the process.

Permitted (p): Which capabilities the process can use or gain. If a capability is in the permitted set, it can be added to the effective set by the process.

Inheritable (i): This set determines which capabilities can be inherited by child processes. If a capability is in the inheritable set of a parent process and also in the permitted set of an executable being run by that process, it can be added to the permitted and effective sets of the child process.<==

Capability Options

cap_chown	Allows changing the owner of any file.
cap_dac_override	Bypasses file read, write, and execute permission checks.
cap_dac_read_search	Bypasses file read permission checks and directory read and execute permission checks.
cap_fowner	Bypasses permission checks on operations that normally require the filesystem UID of the process to match the UID of the file (e.g., chown, chmod).
cap_fsetid	Allows setting the set-user-ID and set-group-ID bits on files; allows setgid to work on a file regardless of the file's GID.
cap_kill	Allows sending signals to any process, bypassing permission checks.
cap_net_bind_service	Allows binding to TCP/UDP sockets below 1024, which are normally reserved for processes with root privileges.
cap_net_broadcast	Allows sending broadcast packets over a network.
cap_net_admin	Allows various network-related operations, such as interface configuration, administration of IP firewall, masquerading, and routing.
cap_net_raw	Allows using RAW and PACKET sockets, providing the capability to craft packets that could manipulate the network, which is potentially dangerous.
cap_ipc_lock	Allows locking memory segments, preventing them from being swapped out to disk.
cap_ipc_owner	Bypasses permission checks for operations on System V IPC objects.
cap_sys_module	Allows loading and unloading kernel modules.
cap_sys_rawio	Allows direct I/O access to hardware devices.
cap_sys_chroot	Allows using the chroot() system call, changing the root directory of a process.
cap_sys_ptrace	Allows using ptrace(), useful for debugging and tracing system calls.
cap_sys_pacct	Allows managing process accounting files.
cap_sys_admin	A broad capability allowing various system administration operations, like mounting filesystems, configuring the kernel, and more.
cap_sys_boot	Allows rebooting or enabling/disabling device power.
cap_sys_nice	Allows raising process priorities and setting real-time scheduling policies.
cap_sys_resource	Allows overriding resource limits.
cap_sys_time	Allows setting the system clock.
cap_sys_tty_config	Allows configuration of tty devices.
cap_mknod	Allows creation of special files using mknod().
cap_lease	Allows establishing leases on arbitrary files (useful for locking).
cap_audit_write	Allows writing to the kernel audit log.
cap_audit_control	Allows control over kernel audit system.
cap_setfcap	Allows setting file capabilities.

Links

Command	Description
ln <source_file> <target_file>	Hard link
ln -s <source_file> <target_file>	Soft link

Hard links

Useful for saving disk space as they share the same data

Deleting one removes the data if it's the last link.

They cannot link across filesystems or to directories.

Soft links

Handy for creating flexible shortcuts that work across filesystems and can point to directories.

The original file needs to exist for the link to work

Deleting the original file breaks the link.

Choosing the Right Option

Use hard links when you want to save disk space and ensure multiple "names" access the same data. But be cautious about deleting links.

Use soft links for flexible shortcuts across filesystems or to directories, but remember they rely on the original file existing.

Key Differences

Attributes	Hard Link	Soft Link (Symbolic Link)
Data	Location	Same as original file, separate file, stores path
Size	Same as original file	Small (contains path only)
Number of Links	Multiple possible	Only one per original file
Works across FS	No	Yes
Links to directories	Not possible	Yes
Deletion impact	Deleting last link removes data	Deleting link doesn't affect original file, deleting original file breaks link