Linux File Communication and Attributes

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Questions and Answers

Which command is used to change the permissions of a file?

chmod (correct)

umask

chown

chgrp

What does the umask command do?

Displays current directory contents

Changes the owner of a file

Changes the group of a file

Sets default permissions for newly created files (correct)

What permissions does the umask value of 027 allow for the owner, group, and others?

Read, write, and execute for the owner, no permissions for group and others

All permissions for the owner, write and execute for the group, no permissions for others (correct)

All permissions for the owner, no permissions for the group, no permissions for others

Read and execute for the owner, write for the group, no permissions for others

Which option is required to change permissions recursively within a directory using the chmod command?

-R Signup and view all the answers

What must you be to change a file’s group using the chgrp command?

The owner of the file and a member of the target group Signup and view all the answers

What is a characteristic feature of symbolic links?

They provide a way to reference files without duplicating data. Signup and view all the answers

How are named pipes (FIFOs) typically used in a UNIX-like operating system?

To facilitate communication between processes. Signup and view all the answers

What effect does the setuid bit have on an executable file?

Grants the permissions of the file's owner during execution. Signup and view all the answers

What does the sticky bit do when set on a directory?

Allows directory deletion only by the file owner. Signup and view all the answers

Why might a user create a named pipe in a filesystem?

To synchronize operations between unrelated processes. Signup and view all the answers

What distinguishes hard links from symbolic links?

Hard links maintain the original file's inode and data structure. Signup and view all the answers

Which type of file would you typically find in the /dev directory?

Character and block device files. Signup and view all the answers

What happens to a file when its last hard link is deleted?

The file is deleted and its data blocks released. Signup and view all the answers

What is the primary function of named pipes?

To enable processes to communicate on the same host Signup and view all the answers

How are symbolic links created in a filesystem?

Using the ln -s command Signup and view all the answers

Which of the following statements about file permission bits is true?

Read bit allows opening and reading the contents of the file. Signup and view all the answers

What does the setuid bit allow when set on an executable file?

Grants access to files and processes normally off-limits to users Signup and view all the answers

Under what condition can files be deleted in a directory with the sticky bit set?

Only the file's owner can delete the file. Signup and view all the answers

What command is used to inspect file attributes such as owner and permissions?

ls -l Signup and view all the answers

What is a characteristic of a symbolic link?

It can reference files across different filesystems. Signup and view all the answers

How do the setgid bits influence newly created files in a directory?

They ensure new files take on the group ownership of the directory. Signup and view all the answers

Which bit has been rendered obsolete and is often ignored by modern systems?

Sticky bit Signup and view all the answers

What happens when a write bit is set on a file?

It permits modification of the file's contents only. Signup and view all the answers

Study Notes

Serial Ports

The first serial port is /dev/tty0
It has a major device number of 4 and a minor device number of 0

Local Domain Sockets

Allow communication between processes
Only accessible from the local host
Referred to through a filesystem object
Examples: syslog and the X Window System
Created using the "socket" system call
Removed using the "rm" or "unlink" system call

Named Pipes

Allow communication between processes on the same host
Also known as FIFO files
Created using "mknod"
Removed using "rm"
Similar in purpose to local domain sockets

Symbolic Links

Point to a file by name
Distinct from the files they point to
Created using "ln -s"
Removed using "rm"
Can refer to files on other filesystems or nonexistent files

File Attributes

Nine permission bits control read, write, and execute access
Three additional bits make up the file's "mode"
Owner and superuser can modify permissions using the "chmod" command.

File Permissions

Only the most specific permissions apply
Regular Files:
- Read bit allows reading the file
- Write bit allows modifying the file contents, but not deleting or renaming it
- Execute bit allows the file to be executed (binaries and scripts)
Directories:
- Execute bit allows entering the directory (search bit)
- Read and execute allows listing directory contents
- Write and execute allows creating, deleting, and renaming files inside the directory

Setuid & Setgid Bits

Setuid (4000) allows programs to access files and processes that would usually be off-limits
Setgid (2000) causes newly created files in a directory to inherit the directory's group ownership

Sticky Bit

Octal value: 1000
Modern systems ignore it when set on regular files.
If set on a directory, prevents deletion or renaming of files unless you are the directory owner, file owner, or superuser

ls Command

Allows inspection of link count, owner, group, mode, size, access/modification timestamps, and file type
Use "ls -l" for detailed info
"ls -ld" for directory info
Directories have at least two hard links: one from the parent directory ("..") and one from the special "." file
"/etc/fstab" lists filesystems that are normally mounted on the system
"umount" detaches filesystems

File Tree Organization

Files are divided by function
The root filesystem includes the root directory ("/") and essential files/subdirectories
Common directories:
- /boot: OS kernel
- /etc: Critical system and configuration files
- /sbin & /bin: Important utilities
- /tmp: Temporary files
- /dev: Devices
- /lib or /lib64: Shared library files
- /usr: Standard programs, manuals, libraries
- /var: Spool directories, log files, accounting info, variable data

File Types

Regular files: Series of bytes
Directories: Named references to files
Character device files: Device drivers present a standard communication interface that looks like a regular file
Block device files: Similar to character devices
Local domain sockets: Communication between processes on the same host
Named pipes: Communication between processes on the same host (FIFO files)
Symbolic links: Point to files by name
The first character of the "ls" output indicates the file type

Regular Files

Consist of a series of bytes
Filesystems don't impose structure
Text, data, executable programs, and shared libraries are stored as regular files
Support sequential and random access

Directories

Contain named references to other files
Created using "mkdir" and deleted using "rmdir" (if empty)
"." refers to the directory itself, ".." refers to the parent directory

Hard Links

File names are stored in the parent directory, not within the file itself
Multiple directories can refer to the same file
Hard links are synonymous with the original file
Filesystem tracks the number of hard links; data blocks aren't freed until the last link is deleted
Cannot cross filesystems
Created using "ln" and removed using "rm"

Character and Block Device Files

Kernel drivers handle the specifics of managing devices
Device drivers present a standardized interface to the kernel that looks like a regular file
These "device files" are rendezvous points for communication with drivers
Identified by major and minor device numbers
- Major device number: Driver responsible
- Minor device number: Specific physical device unit

Other "ls" Options:

"-a": show all files (including hidden ones - "." and ".." are always hidden)
"-l": long listing of files
"-R": recursively applies options to directories
"-t": sort by modification date and time
"-S": sort by file size
"-h": human readable file sizes

chmod: Change Permissions

Only the owner of the file and the superuser can change permissions
Accepts octal notation and mnemonic syntax:
- Octal: chmod 711 myprog (rwx-r-x)
- Mnemonic: chmod u+x myprog (add execute permission for user )
"--reference=filea": copy permissions from filea to fileb
"-R" option: recursively applies permissions to a directory

chown & chgrp: Change Ownership and Group

To change the group, you must be the superuser or the file owner and a member of the target group
chown is now a privileged operation
"-R" flag: recursive application
chown can change owner and group simultaneously using syntax: chown user:group file

umask: Assign Default Permissions

Sets the shell's default permissions (umask)
Inherited by commands run by the shell
Specified as a three-digit octal value representing permissions to take away
Example: umask 027 (remove write permission for group, all permissions for others)

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Description

Test your knowledge on serial ports, local domain sockets, named pipes, symbolic links, and file attributes in Linux. This quiz covers essential concepts regarding inter-process communication and file permission management. Assess your understanding and improve your Linux skills!