Linux essentials

Index

• Package management
  • RPM based
  • Debian based: APT and dpkg
• Command-line basics
  • Command-line syntax
  • Basic commands
  • Command history and completion
  • shell configuration files
  • Environment / shell variables
  • User-defined variables
  • Globbing
  • Quoting
  • Formatting commands
  • Working with options
  • Locate, find, whereis
• Getting CLI help
  • Linux MAN pages
  • Info pages
  • More local documentation

Package management

RPM based

Yum

YUM repos are located in /etc/yum.repos.d/. Unlike APT, YUM has several repo files in the folder. Check out the man pages for YUM and the other package management commands.

command	purpose
yum update	updates the repos and gives you the option of updating the packages pending updates
yum search httpd	searches for that package
yum install $package	to install it
yum check-update $package	to see if a package needs any updates
yum upgrade	upgrade package
yum deplist $package	check package’s list of dependencies
yum clean packages	will remove dependencies that were left behind but are no longer needed
yum remove $package	removes the package
yum list installed	list all installed packages

RPM

| command | purpose | | — | — | | rpm -ipv package.rpm | i means install, p means show progress, and v means verbose | | rpm -q nano | query the package for info (true file name) | | rpm -qi nano | query the package for more info | | rpm -e nano | uninstall the package | | rpm -qR nano | uninstall required packages |

Debian based: APT and dpkg

APT

Uses a sources list located in etc/apt/sources.list

command	purpose
apt-get update	searches the online repos and caches the list of packages for when we do a search via…
apt-cache search $package	searches for a package in the APT cache
apt-get install nginx	install package
apt-get remove nginx	remove package
apt-get remove --purge nginx	to get rid of config files and such
apt-get autoremove [$package]	to remove unneeded packages.
apt-get upgrade	upgrades packages
apt-get -f upgrade	Imstalls dependencies that we’re flagged while attempting to install a Debian package
apt-get dist-upgrade	upgrades the kernel and distribution packages

dpkg

command	purpose
dpkg -i name.deb	Installs Debian package
dpkg --get-selections	shows all installed packages
dpkg --remove $package_name	Removes Debian package
dpkg --purge $package_name	Removes dependencies

Command-line basics

Shells are command-line interpreters that accept commands that are then sent to the OS kernel for processing. See list of popular shells I saved as an img on my iPad. You can use any shell installed on the computer by typing its name on the CLI.

• An interactive shell is one in which you issue commands. A non-interactive shell is one that runs in the background.
• You can have several CLI sessions, accessible via Alt + F1 through Alt + F6.
• You can echo $SHELL
• You can set each user’s shell and see it in /etc/password
• If using bash, You can see your command-line history in .bash_history and complete commands with tab
  • history
  • history 20 show the last 20 commands.
  • !20 execute command # 20
  • !-2 execute the second-to-last command
  • !! execute the previous command
  • !ssh execute the last SSH command
  • !?search? execute the last command with “search” somewhere in it
  • ^original^replacement^ find latest command with original and replace that string with replacement on execution.
    • e.g.: cat /etc/hots, then ^hots^hosts^

Command-line syntax

Programs only run from inside folders indicated in the $PATH variable and not the working directory.

• The letters after - are called flags.
• ls:
  • -a = all (show hidden files)
  • -l = long listing (type and permissions, number of links the file has, owner, group, size in bytes, date modified)
  • -F = Display a slash after a directory, an asterisk after an executable,
  • -S = sort by file size, descending
  • -r = reverse the sorting to ascending. E.g., ls -lrS an @ after each symbolic link.
  • -R = recursively display contents of directories.
  • -t = sort by date modified, desc

Basic commands

Command	Purpose
cd	change directory. By itself, takes you to your home directory.
env	display current user’s environment variables
halt or init 0	shutdown. Note that init works but is deprecated
ifconfig or ip addr	shows NIC configs
netstat	status of the network
reboot or init 1	restart. Note that init works but is deprecated
route	view routing table
shutdown	-H= halt; -P= poweroff; c = cancel pending shutdown; r = reboot
su	substitute user or super user. E.g., su josue or su - to become root
top	list of running apps/processes; top -h gives usage info
uname	print OS name. -n = hostname; -r = kernel’s release; -v = kernel’s version number; -m 32- or 64-bit; -p = processor info; -o = full official name of the OS; -a = all info above
which $program	full path of the application
whoami	current user

Command history and completion

• A user’s command history is kept in ~/.bash_history
• The $HISTFILESIZE env variable shows how many lines will be saved in the history file. a value of 0 means save nothing.
• The $HISTCONTROL env variable shows Bash’s history behavior
• history shows a numbered list of the commands. Rerun a command with !<num>
• You can use the tab key to complete a partially-typed command

shell configuration files

Different shell use different configuration files. Make sure you know which files your Linux distro uses. A system without a GUI puts you in the login shell. It’s important to know which shell your in so you know which configuration file will be used for it.

• login shell: Shell you’re presented with when you log in remotely (e.g., SSH)
  • First file executed is /etc/profile. This file sets default variables for all users.
  • Then these files are executed in order. Once there’s a match the others are ignored even if they exist:
    • ~/.bash_profile
    • ~/.bash_login
    • ~/.profile
    • ~/.bashrc
    • /etc/bashrc
  • ~/.bash_logout is executed when the user logs out
• non-login shell: Shell you’re presented with when you use the terminal application or when you run a script
  • Executes ~/.bashrc, which calls /etc/bashrc
• bash_profile: Login shell that stores user-specific shell preferences
• bashrc: Non-login shell that stores user-specific functions and aliases
• /etc/profile: (check exact name) affects all users

Environment / shell variables

• Variables are placeholders for another value. They can be used in scripts.
• User defined variables: Created by the user
• Environment variables: Created by the OS to configure the system environment. E.g., echo $HOME
• You can view all environment variables with env (not alphabetized) and set (alphabetized)
• Change or create a variable (note there is no space on either side of equal sign:
  • via VAR=VALUE. Example, PATH=$PATH:/var/opt/.
  • export $PATH to make that new value available to users in other shells
  • Configure your bash config files to make this happen every time you start your system.

Common environment variables

variable	description
LOGNAME	username of current user
OLDPWD	previous working directory
OSTYPE	duh
PATH	distro dependent
USER and USERNAME	username of current user
HOST and HOSTNAME	system hostname
ENV	you can type env or set
EUID	UID number of current user
HISTFILE	full path of file
HISTSIZE	size history can grow to

User-defined variables

• Variables cannot start with a number. They can contain - amd _
• Convention is to make variables upper-case.
• Example: THEDUDE="Jeff Bridges" ; export THEDUDE

Globbing

Globbing is the process of using wildcards to expand a search. Globbing stands for global command.

• * = match 0+ of any character
• ? = match 1 of any character
• [Aabc] = match any single character in list
• [^abc] = exclude characters in list
• Examples:
  • ls -l ????.txt search for a four-character text file
  • ls -l [F]*.txt search for all text files beginning with capital F
  • ls -l f[igh][lfz]e*.txt what you’d expect from regex, except that * matches anything 0+ times
  • ls -l [Rr]eport201[0-9]

Quoting

Character	Description	Example
"	allows variable interpolation.	echo "The path is $PATH"
'	does not allow variable interpolation	echo 'The path is $PATH'
\	Escapes special chars	echo "You owe \$5.00"

Formatting commands

• Commands tend to be lower-case
• Spacing doesn’t matter (2 spaces or a tab is OK)
• You can wrap long commands along several lines but will need to escape it. E.g., ls \{enter key} -lah

Working with options

• Command = what to do; options = how to do it; arguments = what to do it with.
• Parameters with a leading - are called options and switch certain parts of the command on/off. ls -la = ls -l -a
• Paramters with no leading - are called arguments.

Locate, find, whereis

Locate

Searches its file database for files or directories the user has access to. Faster than find but doesn’t allow you to indicate the directory.

locate passwd

Find

• Syntax is find $dir [$dir2] {-name | -iname | -size | -mtime | -atime | -ctime}
• You can glob using *, ?, and [].
  • If you glob, you must use single quotes.
• This command will search the directory recursively.
  • You can exclude a directory with -or -iname "$dir_name" prune

Examples:

find . -iname '*keyword*'                   # Match keyword
find / -size +1024                          # greater than size in bytes
find . -mtime -1                            # modified time less than 1 day
find . -atime -1                            # accessed time less than 1 day
find . -ctime -1                            # created time less than 1 day
find . -iname '*.txt' -or -iname "implementations" -prune

whereis

Searches for executables and man page files

whereis cd

Getting CLI help

Linux MAN pages

• Meant as a quick reference for people who already know a command and need to learn certain options. Not meant to be a tutorial.
• Quality can vary significantly from one page to the next
• Man pages have 9 sections. You’ll mostly use section 1, executable programs and shell commands. To see section 5, for example, type man 5 $command
• Use whatis $command to search for man page entries matching that command. E.g., whatis passwd.
• apropos $keyword search man pages for entries containing the keyword.
• You can use / inside a man page to search forward or ? to search backwards
• Man pages are organized like this:
  • Name
  • Synopsis: Brief description of how the command is used, incl. optional parameters (in brackets) and required paramters (underlined). ... means multiple parameters of that type. E.g., ls -la file1 file2
  • Description
  • Options
  • Files
  • See Also
  • Bugs
  • History
  • Author

Info pages

• Similar to man pages, but has hyperlinks (denoted by asterisk.
• Programs from the Free Software Foundation use info pages instead of man pages.
• info $topic

More local documentation

• other ways to get help include an application’s readme file.
• Locations for readme files include:
  • /usr/doc/packagename
  • /usr/share/doc/packagename
  • /usr/share/doc/packages/packagename
• Many programs have help files in PostScript, PDF, or HTML format.
• Configuration files are typically in the /etc directory.
• For RPM packages, try rpm -ql passwd | grep doc or rpm -ql yum | grep README

Reading different file formats

file ext	program used to read them
.1 - .9	man, info, less
.gz or .bz2	gunzip or bunzip2 to decompress, then less to read
.txt	any text editor
.htm, .html	any web browser, often less
.odt	LibreOffice, OpenOffice.org, any word processor
.pdf	.xpdf, Adobe Reader
.tif, .png., .jpg	Gimp

The Linux file system

The Linux file system and the file system hierarchy standard (FHS)

• The Linux file system uses a hierarchy structure to organize data.
• Linux systems have a standard set of subdirectories at the root.

directory	description
bin	executables necessary to run the OS
boot	bootloader files to boot Linux
dev	devices that send/receive data sequentially (printers/mice); devices that are block-oriented (HDs, flash drives)
etc	text-based config files used by the system
home	home folders for users
lib -> usr/lib	code libraries for programs in the bin or sbin directories
lib64	64-bit libraries
media	used by some distros to mount external devices
mnt	used by some distros to mount other external devices
opt	contains files for programs you can install manually
proc	pseudo file system for processes
root	root user’s home directory
sbin	mgmt and config files
srv	where services save their files (e.g., httpd)
sys	hardware within system
tmp	temporary files created by file system
usr	application files
var	Linux variable data and log files

Disk file systems

• ext2: second extended file system. Data is stored in hiearchical fashion (dirs/files). 2 TB is max file size.
• ext3: updated version of ext2. Adds journaling, which records transactions. If the system restarts uncleanly, the entire file system doesn’t need to be checked entirely as with ext2.
• Reiser: similar to ext3 in that journaling makes recovery much quicker.
• ext4: updated version of ext3.

Files and directories

• touch -d "February 1 2017" file.txt: Allows you to specify the modification timestamp.
• mkdir -p newdir/newsubdir/newsubdir2: Create a directory and its parents if they don’t exist.
• rmdir $dir: Can only delete empty directories
• cp -puR srcfile dstfile:
  • p = preserve original ownership
  • u = update (only if src is newer or dst doesn’t exist)
  • R = recursive
• mv srcfile directory/: Move/rename file to indicate directory

Archives and compression

Archives

• tar: Stands for “tape archive.” Combines files but doesn’t compress them. You can pass flags to both archive and compress, since Gzip and Bzip2 are for compressing only. Note that the order of the flags matters for tar command.
  • -c = create archive
  • -f = read the archive from or write the archive to the specified file
  • -t = list archive’s content w/o extracting it
  • -x = extract tarball
  • -v = verbose output (lists files extracted)
  • -z = compress using gzip
  • -j = compress using bzip2 (like gzip but more resources intensive)

Archive (no compression)

Command	Notes
tar -cf tarball.tar dir-to-tar	creates the tarball from dir-to-tar directory
tar -cf tarball.tar file1 file2	creates the tarball from files indicated

Unarchive (no compression)

Command	Notes
tar -tf tarball.tar	show contents of tarball w/o unarchiving
tar -xf tarball.tar	extract tarball
tar -xvf tarball.tar	extract tarball, verbose

Archiving with compression

While not required, it’s best practice to indicate the compression used as part of the file name.

Command	Notes
tar -czf tarball.tar.gz dir-to-tar	use gzip to compress
tar -cjf tarball.tar.bz2 dir-to-tar	use bzip2 to compress

Unarchiving compressed tarballs

Command	Notes
tar -xzf tarball.tar.gz	extract compressed gzipped archive
tar -xjvf tarball.tar.bz2	extract compressed bzip2 archive, verbose

Compression

• zip: Like in Windows. It’s the only command that both compresses and archives. It’s also the simplest b/c it has few options.
  • Examples: Compress with zip and extract with unzip
    • zip file.zip file1 [file2] = create a zipped archive of the specified files
    • zip -r file.zip dir-to-zip
      • -r is required to go into directories and zip their contents, or else you’ll zip an empty directory
    • unzip file.zip
• gzip: Compressed file format. Not for archiving, but for compressing.
  • Compress with gzip and extract with gunzip.
  • File extension is .gz.
  • Note: Gzip deletes the original unless you pass the -c flag, or the -k flag for “keep.”
  • Examples:
    • gzip file.tar = will replace original file
    • gzip -c file.tar > file.tar.gz = will keep original file and create file.tar.gz
    • gzip -k file.tar = same as above
    • gunzip file.tar.gz = uncompress file
• bzip2: Better than Gzip, especially for bigger files. Not for archiving, but for compressing.
  • Compress with bzip2 and extract with bunzip. File extension is .bz2
  • Examples:
    • bzip2 file.tar
    • bunzip2 file.tar.bz2

Searching for and extracting data from files

Viewing text

Command	Purpose
cat	display contents of a file
less -M	reads file with pagination. use / to search fwd and ? to search backwards -M shows lines you’re reading and total, plus percentage -N shows line numbers on the left G goes to beginning and shift + G goes to end
head	read first 10 lines of a file. -n $num_lines
tail	read last 10 lines of a file; -f = follow
find	locates file on system find . -type d find directories find . -type f find files find . -iname "file*. allows globbing

Analyzing text

• grep: searches for a string; allows globbing
  
  • -r = recursive
  • -i = case insensitive
  • -n = show line numbers
  • -w = expression is searched for as word
  • Examples:
    • grep ^Sirloin file1.txt
    • grep -i dhcp /var/log/messages
    • grep -n dhcp /var/log/messages
    • grep -rnw '.' -e 'domo' searches all files in the current folder for the expression
• sort: sorts text alphabetically
  • -r = reverse alphabetically
• cut: Remove text from file and print specified fields to stdout
  • -d = delimiter. E.g., -d" " = space character as the delimiter
  • -f = which field to print (based on the delimiter)
  • Examples:
    • cut -d" " -f 6- = start from field 6 through EOL
• wc: word count. Note that you can specify multiple files.
  • -w = words
  • -l = lines
  • -c = chars

Pipes and regular expressions

You can pipe the output of one command as the input for another command:

grep -i republic plato_republic.txt | less
grep -i republic plato_republic.txt | wc -w

Regular expressions for grep command

Expression	Description	Example
*	0+ repeats of preceding character string or regex	file*
.	any single char (grep)	.cc
?	0+ of proceeding chars	f?le
^	appears at beginning	^.b
$	appears at end	^...$ 3 chars
\b<needle>\b	word boundary (must match exactly)	\bwww\b
[nnn]	one char btw braces	[abc]
[^nnn]	no chars btw braces	[^abc]
[a-z]	any single char in range	[a-x]
[1-90]	any digit between 1-9, and 0	``

I/O Redirection

Redirecting output

Output is normally displayed on the screen but can be redirected to files or to other commands as input.

tail /var/log/messages > logtemp.txt    # redirect stdout
tail /var/log/messages 1> logtemp.txt   # same as above
tail /var/log/messages >> logtemp.txt   # append

cat bogusfile.txt 2> errors.txt         # redirect stderr
cat bogusfile.txt 2>> errors.txt        # append

command 1> outfile.txt 2> errfile.txt   # redirect to separate files

Turning commands into a script

Basic text editing

nano

• ctrl + k: cut line
• ctrl + u: paste line
• ctrl + w: search for text
• ctrl + t: spell check
• ctrl + \: find and replace
• ctrl + g: view help
• ctrl + x: exit

vim

• vimtutor = built in tutorial from beginner to advanced
• Insert mode via i, INSERT, s, o, a
• There are command mode adn command line mode. Enter command line mode from command mode via :
  • v = enter visual mode. V = highlights the line; ctrl + V = visual block
    • y = “yank” or copy highlighted text
  • p = “put” or paste text
  • shift + a = append text at end of line
  • u = undo last change
  • h = move left
  • j = move down
  • k = move up
  • l = move right
  • dw = delete word under cursor
  • dd = deline line under cursor (5dd = delete 5 lines)
  • shift + g = go to bottom of file
  • gg = go to top of file
  • :w = write to disk
  • :wq or x = write to file and quit
  • q! = quit without saving

Shell scripting

• #!/bin/bash = specify an interpreter, (called the shebang)
• Arguments: $1 -> first arg, $2 -> second arg, $? -> exit code/status
• &&: execute command 2 only if command 1 exits normally
• ||: execute command 2 only if command 1 exits abnormally
• You can combine && and || as such:
  rm file1.txt && echo "file deleted" || echo "file not deleted"
• When scripts are first created, they are not executable. Fix this with chmod +x <file name>

Basic if statement

if [ condition ]
then
    command
fi


# example

if ["1" == "1"]
then
    echo "They are the same"
fi

Basic else statement

if [ condition ]
then
    command
else
    command
fi


# example

if [ "$PWD" == "$HOME" ]
then
    echo "You are home."
else
    echo "You are in $PWD."

Loops

for i in {1..10}
do
    echo "$i"
done

Example bash script 1

#!/bin/bash

# My daily routine script

# If user enters "day", show calendar and date
SHOWDAY=$1

if [ "$1" == "day" ]
then
    # Display the calendar
    cal

    # Display the date/time in UTC
    date -u
fi

# Daily greeting
printf "\nHello there, $LOGNAME.\n\n"

if [ "$PWD" == "$HOME" ]
then
    echo "You are home."
else
    printf "You are in $PWD.\n\n"
fi

# Show us what we have to work on today
DOCUMENTS="/Users/rkumar/Downloads/linux-essentials-practice/text-analysis"

for doc in "$DOCUMENTS"/*.txt
do
    echo "$doc"
done

Example bash script 2

Create the script to set variable values based on args

#! /bin/bash
# list contents of a dir and write the output to a file
# usage: ./findlist.sh $LOCATION $FILENAME

LOCATION=$1
FILENAME=$2

if [ -z "$LOCATION" ]
then
    echo "Please provide location argument"
    exit 0
fi

if [ -z "$FILENAME" ]
then
    echo "Please provide a filename"
    exit 0
fi

ls -lh  $LOCATION > $FILENAME
echo
echo "Script is complete and indexed $LOCATION."
echo
echo "###########################"
echo "Displaying contents of $FILENAME"
echo "###########################"
cat $FILENAME

The Linux operating system

Windows, Mac, and Linux differences

• Windows has a lot of proprietary software and active directory.
• Apple uses proprietary hardware and software
• It’s now easier to switch to Linux b/c many applications are web based.
• CLI: Windows has PowerShell and macOS doesn’t have a CLI-only mode

Linux lifecycle management

• Design
• Develop
• Deploy
• Manage
• Retire

Understanding computer hardware

Command	Purpose
cat /proc/cpuinfo	view processor details
free	view RAM stats in bytes -m = show in MB -g = show in GB
dmidecode	show details about motherboard, BIOS, processor, and RAM
lsblk	view all block devices (e.g., HDD) attached to system
df	view free disk space on HDD -h = human readable format
du -h $path	disk usage; human redable, directories only -a = show files
top	show stats on processor, RAM, and running processes

• Hard drives tend to be named sequentially, such as /dev/sda, /dev/sdb, etc.
• Partitions are named sequentially, so partitions on sda will be called sda1, sda2, etc.

Where data is stored

The kernel

• Core of any Linux installation.
• Responsible for managing every piece of softare on a Linux computer, interfacing with the hardware.
• The kernel launches /sbin/init, and init in turn launches child processes.
• Linux manages these processes in the processes table, which we can access via ps and top.

Linux processes

• Every process has a PID.
• Every parent process has parent ID (PPID)
  • The two parent processes are 1) systemd and 2) kthreadd
• The kernel supplies process information to the /proc directory so it can be available to the ps, top, and free commands.
• We can use ps to identify running processes. Note that this command provides a static snapshot.
  • -u $username shows processes for that username
  • -e shows every process running from all users
  • -H show hierarchy of processes via indented output. E.g., ps -eH
  • --forest also shows process hierarchy. E.g., ps -e --forest
  • -f shows full format listing (all arguments a command is using while running). E.g., ps -ef --forest
  • ps -u josue --forest shows parent/child relationships for processes.
  • ps u U josue gives CPU and memory %.
  • ps aux the u adds the username column. There’s so much output it’s typically more practical to grep.
  • kill -9 $PID will kill a process
• top is dynamic, as opposed to ps, which provides a static snapshot.
  • -h or ? will display CLI usage info and exit
  • After running top…
    • k will prompt for the PID of the process to kill.
    • M sort by memory usage
    • P sort by CPU usage (default)
• free generates a report on the system’s memory status using KB
  • The Mem: line shows total RAM stats
  • The -/+ buffers line shows the total memory used by the programs
  • Swap: is hard disk space used as a adjunct to RAM.
  • The -h flag shows the information in human-readable measurements (MB, GB)

syslog, klog, dmesg

• Most system logs are stored in /var/log/
• Logs are closed daily and retained for several days
• Reading most system log files requires root privileges
• boot.log records events from when the system boots
• messages is the main log file
• secure is the file that logs when users elevate their privileges or attempt/fail to log in
• grep sshd /var/log/*
• klogd manages messages from the kernel separate from other programs.
• dmesg will display messages from the kernel. This helps with tshoot of hardware or driver issues.

Networking

Basic networking

Important network tools

Tool	Purpose
ping -c $num	testing connectivity
dig	dig www.pluralsight.com -t A
nslookup	nslookup -query=A www.pluralsight.com
netstat	list network connections
route	current route/netwk settings
host $fqdn	test DNS resolution
traceroute	trace packet route
ifconfig	current network settings
ip addr [show]	current IP addr and network settings

• You set up DNS information in /etc/resolv.conf, but in some distros you’re not supposed to edit this file.
• You can see CentOS network config in /etc/sysconfig/network-scripts/ifcfg-ens33 or some other ifcfg… file.

Static IP address

• Edit /etc/sysconfig/network-scripts/ifcfg-ens33
  • BOOTPROTO=”static”
  • IPADDR="$addr"`
  • NETMASK="$mask"`
  • NETWORK="$subnet_id"`
  • You can use CIDR notation on the IP address and omit the NETMASK
  • Remember to set DNS information in /etc/resolv.conf
  • You add routes another way

Routes

• ip route show shows the routes
• route older method of showing routes
• netstat -r same output as the route command, including routes to leave the LAN
• Always set your destination gateways as IP addresses, not FQDNs.
• Add routes via route add -net $ntwk_id netmask $mask gw $rtr_addr
• Remove routes via route del -net $ntwk_id netmask $mask gw $rtr_addr
• Route add default gw $ip_addr
• The DNS server used is indicated in /etc/resolv.conf

Other commands

Command	Description
netstat -a	Lists listening & non-listening sockets
netstat -i	Stats about the network interfaces
netstat -l	Lists listening sockets
netstat -s	Summary for each protocol
netstat -r	Equivalent to route

Basic security and user types

Root and standard users

• Only the user and root can access the user’s files.
• finger $username gives info on a user (login, directory, name, and shell)
• id $username gives user ID, group ID, group memberships
• /etc/passwd has list of users who can authenticate locally. Each line indicates the user, the user’s pw (legacy field), UID, GID for default group, full name or comment, home dir, and default shell
• /etc/shadow has list of user passwords. Each line has the username, hashed pw, last modified field in Unix epoch, max days before a password must be changed, days ahead of max when the user will be prompted to change the password, the days to wait to disable the account if the password remains expired, and the expired field.
• /etc/sudoers has a list of sudoers
• /etc/group shows the group, password for the group, GID, and list of users who are members
• pwck checks whether passwd and shadow are in sync.
• pwconv adds any missing users from etc to shadow.
• Root exists to perform administrative tasks and can therefore access all files.
• su or su - let’s you become Root. su - username gives us a shell as that user, with their PATH var.
• sudo $cmd is a per-command way to elevate privileges.
• who = who is logged in
• W shows logged in users and their processes.
• who -b last boot time
• who -m whostname and user associated with it
• who -r our current run level
• who -q number of users logged in
• who -a all of the above
• last [$username] who logged in, when, and how, in reverse chronological order

Creating users and groups

• Every user acct has a UID and a textual username.
• Different users could have the same UID and therefore identical rights to the same files. You should never do this.
• id will show the current user’s UID and GID. You can also type id $username
• groups $username shows the group memberships.
• groupadd <grp-name> = add a new group
• useradd [-G $GID] -m -c "John Doe" jdoe = add a new user. This command pulls defaults from /etc/default/useradd
  • -m = create home dir
  • -c = comment; usually the user’s full name
• userdel -r jdoe = delete user and home folder
• sudo passwd $username = change user’s password.

Managing file permissions and ownership

File and directory permissions

☁  shell-scripting  ll
total 24
drwxr-xr-x  5 rkumar  staff   160B May 21 16:22 ./
drwxr-xr-x  5 rkumar  staff   160B May 21 08:48 ../
-rwxr-xr-x  1 rkumar  staff   546B May 21 16:17 daily.sh*
-rwxr-xr-x  1 rkumar  staff   516B May 21 16:22 indexer.sh*
-rw-r--r--  1 rkumar  staff   1.8K May 21 16:22 test1.txt

• In the output above, the columns on the left indicate the user, group, and global permissions.
• Permissions can be shown via symbolic (the letters above) or octal notation.
  • r = 4
  • w = 2
  • x = 1
  • e.g., daily.sh has octal values 755

Modifying permissions

• chmod = change mode of a file or directory, affecting permissions
  • chmod u=rwx,g=rw,o=r $file_name
  • chmod o-rx daily.sh = remove read and execute permissions from others
  • chmod -R o-rx shell-scripting/* = recursively alter permissions for files in a directory, but not the directory itself
    • Applying the command to the directory instead of including /* also alters the directory.
  • chmod 600 test1.txt = modify permissions on the file with rw permissions for the user and no permissions for the group or others
• chown $file_or_dir = change ownership of a file/directory
  • chown $username:$group $file
    • You can omit the colon and the group if you’re only changing the user. chown $username $file
    • You can omit the user if you’re changing the group membership: chown :$group $file
    • Only root can change the user who owns a file
• chgrp = change group ownership of a file/directory

Special directories and files

Symbolic links

• Symlinks are similar to windows shortcuts. They reference the path to a file, not the file itself.
• If the original file/dir is moved, the symlink breaks.
• ln -s $src_name $link_name one convention is to append .lnk to the end of the symlink name
• unlink $link_name removes the symlink
• Symlinks display an l in the file descriptor column of the ls -l output.
• Hard links are another pointer to the exact data on the hard disk. Deleting only one doesn’t delete the file.
  • ln $src_file $link_name

Special files and directories, and the sticky bit

• /var/tmp: Has temp files that do not get deleted on reboot
• /tmp: Has files that get deleted upon reboot
  • Files in this directory have the sticky bit set, meaning that only users who created a file can delete that file even if everything has rwx permissions for this directory. This cam be seen via ls -ld /tmp, which gives drwxrwxrwt. 8 root root 211 May 23 18:22 /tmp
• There are two ways to apply the sticky bit to a directory:
  • chmod o+t $dir_name
  • chmod 1777 $dir_name the 1 denotes the sticky bit. To remove it, use chmod 777 $dir_name, where the absence of the 1 implies a zero (chmod 0777 $dir_name)