Beginner Linux Flashcards
Who developed UNIX?
Ken Thompson and Dennis Ritchie of Bell Laboratories
It was later rewritten in C to make it more portable.
Who created GNU?
Richard Stallman
“GNU’s Not Unix”
What is the GPL?
The GNU General Public License. A free software license
The most important piece in the operating system
The kernel
What is the primary job of the kernel?
It allows the hardware to talk to the software
Who developed the Linux kernel?
Linux Torvalds
What year was the Linux kernel developed?
1991
What is GNU known for?
The Free Software Movement. A campaigns to win for the users of computing the freedom to run, copy, distribute, study, change and improve free software.
What does the term Linux actually refer to?
The Linux kernel
Why are distributions referred to as “Linux operating systems”?
Because they use the Linux kernel
A Linux system is divided into what three main parts?
- Hardware
- Linux kernel
- User space
What are the most popular Linux distributions?
Debian Red Hat Enterprise Linux Ubuntu Fedora Linux Mint Gentoo Arch Linux
A distribution used mostly as an enterprise server OS
Red Hat Enterprise Linux (RHEL)
A Debian-based operating system developed by Canonical.
Ubuntu - One of the most popular Linux distributions for personal machines
An upstream RHEL operating system
Fedora - RHEL gets updates from Fedora after thorough testing and quality assurance
A distribution based off of Ubuntu
Linux Mint
A flexible OS made for advanced users
Gentoo
A lightweight and flexible Linux distribution driven 100% by the community
Arch Linux
Created by a community that promotes the use of Linux everywhere, working together in an open, transparent and friendly manner as part of the worldwide Free and Open Source Software community
openSUSE
A program that takes your commands from the keyboard and sends them to the operating system to perform
The shell
Almost all Linux distributions will default to what shell?
bash (Bourne Again shell)
Common shells
bash, bourne, korn, ksh, zsh, tsch
What is everything in Linux?
Everything in Linux is a file
How is every file organized in Linux?
In a hierarchical directory tree
The first directory in the file system is aptly named?
The root directory
The location of files and directories are referred to as?
paths
How to print working directory showing you which directory you are in with a path from root?
pwd
What command let’s you move around the filesystem?
cd (change directory)
What are the two ways to specify a path?
- Absolute path
2. Relative path
The path from the root directory
Absolute path
The path from where you are currently in filesystem
Relative path
The root directory is commonly shown as what?
A slash /
Shortcut to the directory you are currently in
A single period .
Shortcut to the parent directory
A double period ..
Shortcut to the user’s home directory
A tilde ~
Shortcut to the previous directory
A hyphen -
Command to list directory contents and show you detailed information about the files and directories you are looking at
ls
Not all files in a directory will be visible. Filenames that start with what are hidden?
How Filenames that start with a period . are hidden
How do you view hidden files?
Using the -a flag on the ls command (a for all)
$ ls -a
This flag on the ls command will show you detailed information
$ ls -l
What does the Touch command do?
Touch allows you to create new empty files.
Touch is also used to change timestamps on existing files and directories
What does the file command do?
It’s used to find out what kind of file a file is
A simple command to display file contents but it can also combine multiple files and show you the output of them.
cat command, short for concatenate
It’s only meant for short content
Displays text in a paged manner, so you can navigate through a larger text file page by page
less command
There are commands to navigate through less e.g. page up and down, q to quit, g and G to move to the beginning and end of the file
Lists the history of the commands that you previously entered
history
How to run the same command you did before
Press the up arrow
What does !! do?
Runs the previous command without typing it again
How to use the reverse search command?
ctrl-R and start typing parts of the command you want and it will show you matches and you can just navigate through them by hitting the ctrl-R key again
How to clear up your display
clear command
If you start typing the beginning of a command, file, directory, etc and hit the Tab key what happens?
will autocomplete based on what it finds in the directory you are searching
A character that can be substituted for a pattern based selection, giving you more flexibility with searches
A wildcard
Copy files
cp
Wildcard used to represent all single characters or any string
*
Wildcard used to represent one character
?
Wildcard used to represent a character from a group
[ ]
How to recursively copy the files and directories within a directory?
With the recursive flag and the cp command
$ cp -r
How can you ensure that a file you are copying isn’t accidentally overwritten with a file of the same name?
With the interactive flag to prompt you before overwriting a file
$ cp -i
Used for moving files and also renaming them
mv
What does the -b flag do when used with the mv command?
$ mv -b directory1 directory2
It will make a backup of that file and it will rename the old version with a ~
What does mkdir do?
Make Directory
How can you create directories and subdirectories at the same time?
WIth the -p parent flag
$ mkdir -p directory/subdirectory
The command that is used to delete files and directories.
rm
Does Linux have a trash can that you can fish out removed files?
No
What are some safety measures to prevent files from accidental deletion?
Write-protected files will prompt you for confirmation before deleting them
What flag tells rm to remove all files, whether they are write protected or not, without prompting the user?
The -f or force flag
$ rm -f file1
What rm flag will give you a prompt on whether you want to actually remove the files or directories?
The -i interactive flag
$ rm -i file1
How can you remove a directory and remove all the files and any subdirectories it may have?
With the -r recursive flag
$ rm -r directory1
How can you remove an empty directory?
With the rmdir command
$ rmdir directory
A command that allows you to specify a directory to search, and what you’re searching for
The find command
$ find /home -name pic.jpg
$ find /home -type d -name MyFolder
type -d is for directory
A built-in bash command that provides help for other bash commands (echo, logout, pwd, etc)
help
$ help echo
For executable programs, it’s a convention to have a help option
–help
$ echo –help
How to view the manuals for a command?
man for the man pages
$ man ls
What if you are ever feeling doubtful about what a command does?
Use the whatis command
$ whatis ls
What can you do if a command is too repetitive to type over and over?
Create an alias
$ alias foobar=’ls -la’
Alias are temporary and won’t persist after reboot. How can you make an alias permanent?
Add the alias to a file like ~/.bashrc
How to delete an alias?
With the unalias command
$ unalias foobar
How can you exit from the shell?
- ) $ exit
- ) $ logout
- ) With a terminal GUI, you can just close the terminal,
What are the I/O (input/output) streams?
- stdout (Standard Out)
- stdin (Standard In)
- stderr (Standard Error)
What are the I/O streams used for?
Processes use I/O streams to receive input and return output
How can we change the default output behavior of a command line program?
By using I/O redirection with a redirection operator that allows us the change where standard output goes
What does the > operator do?
The > is a redirection operator that allows us the change where standard output goes. It allows us to send the output to a file instead of the screen.
If the file does not already exist it will create it for us. However, if it does exist it will overwrite it.
What does the»_space; operator do?
The»_space; is a redirection operator that allows us the change where standard output goes. It allows us to send the output to a file instead of the screen.
This will append output to the end of the file, if the file doesn’t already exist it will create it.
What is the < operator used for?
< is the stdin redirection operator
What is a file descriptor?
A file descriptor is a non-negative number that is used to access a file or stream
What are the file descriptors for stdin, stdout and stderr?
0, 1, and 2 respectively
What is the redirection operator 2> file.txt doing?
Redirecting stderr output to file.txt rather than the screen
What does 2>&1 achieve?
2>&1 sends stderr to whatever stdout is pointing to
E.g. $ ls /fake/directory > file.txt 2>&1
Outputs stderr and stdout to file.txt
What does &> achieve?
Redirects both stdout and stderr to a file
E.g. $ ls /fake/directory &> file.txt
How can you get rid of stderr messages completely?
Redirect stderr to /dev/null
E.g. $ ls /fake/directory 2> /dev/null
How can get the stdout of a command and make that the stdin to another process?
Using the pipe operator ‘|’.
E.g. $ cat file.txt | less
How can you write the output of a command to two different streams?
Using the tee command.
E.g. $ ls | tee file.txt
Will print the output of ls to the screen and to file.txt
What command outputs a whole lot of information about the environment variables you currently have set?
env
What are environment variables?
Variables that contain useful information that the shell and other processes can use
What is the PATH environment variable?
It contains a list of paths separated by a colon that your system searches for binaries when it runs a command
What is the cut command?
The cut command extracts portions of text from a file
cut by characters with -c
cut by fields with -f
change field delimiter with -d
Similar to the cat command, but it can merge lines together in a file
The paste command with the -s flag
The -d fag can change the delimiter from the default TAB
How can you views the first couple of lines in a text file?
Using the head command you see the first 10 lines
You can also modify the line count using the -n flag
e.g. $ head -n 15 /var/log/syslog
How can you view the last couple of lines in a text file?
The tail command lets you see the last 10 lines of a file
You can also modify the line count using the -n flag
e.g. $ tail -n 10 /var/log/syslog
The -f (follow) flag will follow the file as it grows showing new entries
What command is used to convert TABs to spaces?
expand
What command can convert back each group of spaces to a TAB?
unexpand
This command allows you to merge multiple files together by a common field
join
This command will split a file into different files. By default it will split them once they reach a 1000 line limit.
split
This command is useful for sorting lines in a file
sort
The -r flag will reverse sort
The -n flag will sort by numerical value
This command allows you to translate a set of characters into another set of characters
tr (translate)
Let’s say you had a file with lots of duplicates. How can you remove them?
uniq (unique)
- c counts occurrences
- u shows only unique entries
- d shows only duplicate entries
What is the biggest weakness of the uniq of the command?
uniq does not detect duplicate lines unless they are adjacent so it should be used along with the sort command
This command shows the total count of words in a file
wc
It display the number of lines, number of words and number of bytes or to see a certain field, use the flag -l, -w, or -c respectively
How can we number the lines of a file?
nl (number lines)
What command allows you to search files for characters that match a certain pattern?
grep
Search case insensitive with the -i flag
You can even use regular expressions in your pattern
A powerful tool to do pattern based selection
Regular Expressions
The two most popular text editors that are installed by default on most Linux distributions
vim and emacs
What does vim stand for?
vi (Improved)
An editor that you can do all your code editing, file manipulation, etc all within
emacs
Why do traditional operating systems have users and groups?
They exist solely for access and permissions
What is a user’s home directory?
A directory where user specific files get stored. This is usually located in /home/username
What’s the difference between user ids and usernames?
usernames are the friendly way to associate users with identification, but the system identifies users by their UID
What are groups in Linux?
Sets of users with permission set by that group.
They are identified by the system with their group ID (GID)
What are system daemons?
They continuously run processes to keep the system functioning
The most powerful user on the system
The superuser root that can access any file and start and terminate any process
If root access is needed you can run a command as root using what command?
sudo (superuser do)
This command will “substitute users” and open a root shell if no username is specified
su (substitute users)
if you need to run commands as the superuser should you use su or sudo?
sudo
It’s much easier to make a critical mistake running everything in root using su
What file called the lists users who can run sudo?
/etc/sudoers file
You can edit this file with the visudo command
What is the visudo command used for?
Editing the /etc/sudoers file that lists users who can run sudo
This file shows you a list of users and detailed information about them
/etc/passwd
There are many fields separated by colons that tell you additional information about the user
Why are there users on a Linux system?
Users are really only on the system to run processes with different levels of permissions
What is the vipw tool used for?
You can edit the /etc/passwd file by hand with the vipw tool but it’s better to use tools such as useradd and userdel
This file is used to store information about user authentication. It requires superuser read permissions.
/etc/shadow
In most distributions user authentication doesn’t rely on just the /etc/shadow file. There are other mechanisms in place such as PAM (Pluggable Authentication Modules).
This file allows for different groups with different permissions.
/etc/group
You can use what commands to add, delete or change the password of a user if you are root?
useradd
userdel
passwd
There are what four parts to a file’s permissions?
1.) filetype: d for direcotry or - for regular file
The next three parts of the file mode are the permissions (3 bits each).
- ) user permissions
- ) group permissions
- ) other permissions (everyone else)
There are 3 bits to represent user permissions. What do these characters represent?
r: readable
w: writable
x: executable (basically an executable program)
- : empty
Changing permissions can easily be done with what command?
chmod
You can add or remove permissions with a + or -
Symbolic way:
e. g. $ chmod u+x myfile
e. g. $ chmod u-x myfile
Add and remove executable permission bit on the user set
How can we change permissions all at once instead of using r, w, or x to change a single permission
By using the numerical format
4: read permission
2: write permission
1: execute permission
numerical way:
e.g. $ chmod 755 myfile
7 = 4 + 2 + 1, so 7 is the user permissions and it has read, write and execute permissions
Why should we take precaution when using the chmod command?
You could potentially expose a sensitive file for everyone to modify
You can modify the group and user ownership of the file using what two commands?
chown and chgrp
e.g. $ sudo chgrp whales myfile
This command will set the group of myfile to whales
How can you can set both the user and group at the same time?
Add a colon and group name after the user and set both at the same time
e.g. $ sudo chown patty:whales myfile
How do you change that default set of permissions used when creating a file?
With the umask command
Instead of adding permissions though, umask takes away these permissions
e.g. $ umask 022
All user access (7 - 0 = 7), but no write access for group and other users (7 - 2 = 5 for read and execute)
What allows a user to run a program as the owner of the program file rather than as themselves?
Set User ID (SUID)
Set User gives us a new permission bit - s.
When a file has this permission set, it allows the users who launched the program to get the file owner’s permission as well as execution permission.
e.g. When a user is running the password command, they are running as root because of SUID and that’s why we are able to access a protected file like /etc/shadow
e. g. $ sudo chmod u+s myfile — Symbolic way
e. g. $ sudo chmod 4755 myfile — Numerical way
The numerical representation for the user bit is 4
What allows a program to run as if it was a member of that group?
Set Group ID (SGID) similar to SUID
e. g. $ sudo chmod g+s myfile
e. g. $ sudo chmod 2555 myfile
The numerical representation for the group bit is 2
How many UIDS are associated with every process?
Three
- ) effective user ID - used to grant access rights to a process
- ) real user ID - the actual ID of the user that launched the process
- ) saved user ID - allows a process to switch between the effective UID and real UID
How can we ensure that only the owner or the root user can delete or modify the file?
By using the the sticky bit
e. g. $ sudo chmod +t mydir
e. g. $ sudo chmod 1755 mydir
The numerical representation for the sticky bit is 1
Another name for the programs that are running on your machine
Processes
Processes are managed by the kernel and each process has a process ID (PID). A process is the system allocating memory, CPU, I/O to make the program run.
This command gives you real time information about the processes running on your system instead of a snapshot
top
This command prints a list of running processes
ps
What is the TTY field in the ps output?
The TTY is the terminal that executed the command
What are the two types of terminals?
- ) terminal devices
- ) pseudoterminal devices
A pseudoterminal is what we’re used to working in. They emulate terminals with the shell terminal window and are denoted by PTS under TTY in the ps command
If you were running a program on your shell window such as find and you closed the window what happens?
Your process would also close because processes are usually bound to a controlling terminal
What types of processes are not bound to a controlling terminal?
Processes that run in the background like daemon processes, which are special processes that are essentially keeping the system running
They are denoted by ? under TTY in the ps command
How are new processes created?
An existing process clones itself using the fork system call. The new process is given a process ID (PID) and a parent process ID (PPID). The execve system call is used to launch a new program, which frees up the cloned memory resources, and the kernel re-allocates new memory resources for the new program.
What is the process that spawns all other processes?
When the system boots up, the kernels creates a process called init with a PID of 1. The init process can’t be terminated unless the system shuts down.
What occurs when a process is terminated?
- ) _exit system call which frees up the resources that process was using
- ) The kernel receives a termination status from the process (a status of 0 means that the process succeeded)
- ) The parent process has to acknowledge the termination of the child process by using the wait system call to check the termination status of the child process
What is an orphan processes?
When a parent process dies before a child process, the kernel knows that it’s not going to get a wait call, so instead it makes these processes “orphans” and puts them under the care of init. Init will perform the wait call to shutdown the process
What are zombie processes?
A child process terminates, but the parent process hasn’t called wait yet.
A notification to a process that something has happened.
A signal
They are basically ways processes can communicate
Some of the most common signals?
SIGHUP or HUP or 1: Hangup
SIGINT or INT or 2: Interrupt
SIGKILL or KILL or 9: Kill
SIGSEGV or SEGV or 11: Segmentation fault
SIGTERM or TERM or 15: Software termination
SIGSTOP or STOP: Stop
Numbers can vary with signals so they are usually referred by their names
What signal terminates processes?
kill
e.g. $ kill 12445 (PID)
By default it sends a TERM signal but you can also specify a signal with the kill command
e.g. $ kill -9 12445 (SIGKILL along with kill command)
Differences between SIGHUP, SIGINT, SIGTERM, SIGKILL, SIGSTOP?
SIGHUP - Hangup, sent to a process when the controlling terminal is closed
SIGINT - Is an interrupt signal, so you can use Ctrl-C and the system will try to gracefully kill the process
SIGTERM - Kill the process, but allow it to do some cleanup first
SIGKILL - Kill the process, and don’t cleanup
SIGSTOP - Stop/suspend a process
Processes use the CPU for a small amount of time called what?
A time slice
Process scheduling of time slices is handled by the kernel
What is Niceness?
A way to influence the kernel’s process scheduling algorithm with a nice value. Processes have a number to determine their priority for the CPU. High numbers are low priority while low or negative numbers are high priority.
How can you change the niceness level?
The nice command is used to set priority for a new process
e.g. $ nice -n 5 apt upgrade
The renice command is used to set priority on an existing process
e.g. $ renice 10 -p 3245
A Linux process can be in a number of different states. What are the most common states?
R: running or runnable, it is just waiting for the CPU to process it
S: Interruptible sleep, waiting for an event to complete, such as input from the terminal
D: Uninterruptible sleep, processes that cannot be killed or interrupted with a signal, usually to make them go away you have to reboot or fix the issue
Z: Zombie, terminated processes that are waiting to have their statuses collected
T: Stopped, a process that has been suspended/stopped
Everything in Linux is a file, even processes. Where is process information stored?
In a special filesystem known as the /proc filesystem
The /proc directory is how the kernel views the system. There is a lot more information here than what you would see in ps.
What is Job Control?
We can control how our processes run with jobs so that we don’t lose control of our shell while waiting on a slow process to complete.
Appending an ampersand (&) to the command will run it in the background so you can still use your shell.
What does the jobs command do?
Allow you to view the jobs you just sent to the background
If you already ran a job and want to send it to the background how can you do that?
First suspend the job with Ctrl-Z, then run the bg command to send it to the background.
How can you move a job from the background to the foreground?
Run the fg command along with a specific job ID
e.g. $ fg %1
Leaving off the id will bring back the most recent background job
How can you kill background jobs?
Using the kill command and the job id
e.g. $ kill %1
What do package managers do?
Install and maintain the software on your system
What are the two most common variety of packages?
Debian (.deb) (Debian, Ubuntu, LinuxMint, etc.)
Red Hat (.rpm) (RHEL, Fedora, CentOS, etc.)
What are packages?
Software (Chrome, Photoshop, etc.) consisting of lots of files that have been compiled into one. A package.
People that create software packages are also known as?
upstream providers
Who reviews, manages and distributes software in the form of packages?
package maintainers
A central storage location for packages
package repositories
What is an archive of files?
A single file known as an archive contains many files inside of them. Archives have file types such as .rar and .zip.
A program used to compress and uncompress files in Linux
gzip and gunzip
Files compressed with gzip end in .gz
e. g. $ gzip mycoolfile
e. g. $ gunzip mycoolfile.gz
What is the difference between gzip and tar?
gzip can’t add multiple files into one archive
What is this command doing?
$ tar cvf mytarfile.tar myfile1 myfile2
Creating an archive with tar
c - create
v - be verbose and let us see what it’s doing
f - the filename of the tar file has to come after this option
What is this command doing?
$ tar xvf mytarfile.tar
Unpacking archives with tar
x - extract
v - be verbose and let us see what it’s doing
f - the file you want to extract
How can we use tar and gzip together with a file like mycompressedarchive.tar.gz,?
First remove the compression with gunzip and then you can unpack the tar file. Or you can alternatively use the z option with tar, which just tells it to use the gzip or gunzip utility.
e. g. $ tar czf myfile.tar.gz creates a compressed tar
e. g. $ tar xzf file.tar uncompress and upack the file
eXtract all Zee Files
How can you install direct packages?
You can use the package management commands
rpm for .rpm
dpkg for .deb
Two of the most popular package management systems
yum for redhat
apt for debian
What is the difference between (yum and apt) and (rpm and dpkg)?
yum and apt also include installing package dependencies
When building from source, what should you use instead of make install?
$ sudo checkinstall
This command will essentially “make install” and build a .deb package and install it. This makes it easier to remove the package later on.
