Review notes for RHCSA

This document mainly contains command snippets of the various CLI tools used for the exam

unrelated helpful snippets

echo "password" | sudo -S dnf update -> pipes password into sudo commandfrom STDIN

output redirection & piping

STDIN: - < or 0< - destination: keyboard - File Decriptor: 0

STDOUT: - > or 1> - destination: monitor - File Decriptor: 1

STDERR: - 2> - destination: monitor - File Decriptor: 2

redirection Examples

>> or 1>> -> redirects STDOUT in append mode

2>&1 -> redirects STDERR to same dest as STDOUT

ls /home/ryan 1> /dev/tty1 -> redirects output to console on server

ls /home/ryan 2>&1 > /dev/null -> redirects STDERR to same location as stdout and sends to /dev/null device file.

chmod

SUID -> chmod u+s , chmod 4XXX

SGID -> chmod g+s , chmod 2xxx /somedir

Sticky Bit -> chmod +t , chmod 1XXX

history

history -c -> clears current history

history -w -> remove current history and .bash_history file

history -d <number> -> deletes item from history

umask

F= Files , D=Directories

0 -> F: Read and write , D: everything

1 -> F: Read and write , D: Read and write

2 -> F: Read, D: Read and Execute

3 -> F: Read , D: Read

4 -> F: Write , D: Execute

5 -> F: Write , D: Write

6 -> F: Nothing , D: Write

7 -> F: Nothing, D: Nothing

Find

find / -user <user>

find / -group <group>

Variables

$LANG=en_US.UTF-8 -> sets language for system , using UTF-8 encoding

env -> shows environment variables

Bash environment

/etc/profile -> generic file , processed by all users upon login

/etc/bashrc -> processed by subshells

~/.bash_profile -> user-specific login shell variables

~/.bashrc -> user-specific file , subshell variables can be defined

man

man -k <keyword

apropos

mandb -> updated mandb

df -Th -> Shows available disk space on mounted devices , lists disk type

findmnt -> shows mounts and relationships , lists mount options

ls

ls -l -> view permissions set on each file

ls -a -> view all files , including hidden files or “dot” files

ls -lrt -> view most recently modified files

ls -d

ls -R

tar

tar -rvf /root/archive.tar /etc/hosts -> adds /etc/hosts file to the archive tar -tvf archive.tar -> list contents of archive , shows permissions on each file in archive as well.

tar -uvf homes.tar /home/user/ -> updates archive with newer versions of existing files

tar -xvf random.tar -C /tmp/ -> Extracts tar archive to temp dir with -C option.

tar -xvf random.tar random/file.txt -> extracts specific file from archive

tar -xvf random.tar -C /tmp/ random/file.txt -> extracts speicifc file to specified directory

tar -cvJf /tmp/home.tar /home/user -> Create xz archive

tar -xvJf /tmp/home.tar /home/user -> Extract xz archive

tar -cvjf /tmp/home.tar /home/user -> Create bzip2 archive

tar -xvjf /tmp/home.tar /home/user -> Extract bzip2 archive

tar -czvf /tmp/home.tar /home/user -> Create gzip archive

tar -xzvf /tmp/home.tar /home/user -> Create gzip archive

gzip & bzip2 & xz

bunzip2 -> decompresses bzip2 archive

gunzip -> decompresses gzip archive

cat & tac

cat -> outputs contents from beginning to end of file

tac -> outputs contents of file from end to beginning

head & tail

head -> shows first 10 lines in file by default

tail -> shows last 10 lines in file by default

tail -<x> file or tail -n<x> file -> outputs last x lines of file

head -<x> file or head -n<x> file -> outputs last x lines of file

tail -f file -> outputs last lines of file ,updates when new lines are added.

cut

cut -d : -> specifies delimeter , in this case it’s the “:” character

cut -d : -f 1 -> “-f” specifies which field to filter out , “:” is used as delimeter.

tail /etc/passwd | cut -d : -f 1 -> outputs the last 10 users listed in the /etc/passwd file

sort

sort -> by default sorts content in byte order , aka the order they appear in the ascii text table

sort -n -> sorts in numeric order from least to greatest

sort -nr -> sorts in numeric order from greatest to least

sort -k3 -t : /etc/passwd -> sorts the third column in /etc/passwd in numeric order

wc

wc -> outputs number lines , words , and characters

ps aux | wc -> outputs total processes are running on the system as each line in the output represents a running process.

grep & regex

grep -i -> case insensitive

grep -v -> show only lins that do not contain the regular expression

grep -r -> searches files in the current directory and subdirectories

grep -e -> used for lines matching more than one regular expression

grep -A<number> -> shows “number” of lines matching regular expression

grep -B<number> -> shows lines before the matching regular expression

grep <keyword> <file> -> grep syntax

grep ^anna /etc/passwd -> shows lines that start with text “anna”

grep nologin$ /etc/passwd -> shows all lines that end with nologin

grep ':*/var' /etc/passwd -> “*” matches zero or more of the previous character.

grep -E <expr> <file> -> tell grep to interpet extended grep expressions grep '^#' /etc/services -> shows lines that start with “#” in file

grep -v '^#' /etc/services -> shows lines that do not start with “#”

<text>$ -> shows lines that end with text before “$”

^something -> Shows lines that start with text after “^”

r.t -> wildcard , searches for one character that matches the single character that matches the pattern

Note: put regex expression in quotes if you don’t wanna escape characters avoid misinterpretation

+ -> indicates character should occur one or more times

? -> indicates characters should occur zero or one times

\{2\} -> matches exactly two of the prevous character

\{1,3\} -> matches a minimum of one and a mx of three of the previous character

(..) -> parentheses are used to group multiple characters in the expression

awk

awk -F : '{ print $4 }' /etc/passwd -> prints out the 4th column of the /etc/passwd file , uses “:” as a delimeter.

awk -F ' /ryan/ { print $3 }' /etc/passwd -> Gets the UID of the user “ryan” from the /etc/passwd file

ps aux | awk -F" " '{print $2"\t"$4} -> Prints out the PID and MEM columns from ps aux command.

sed

sed -n 5p /etc/passwd -> prints fifth line from /etc/passwd file

sed -i s/<old-text>/<new-text>/g -> replaces “old-text” in file with “new-text” , “g” means changes are added to all matching lines

sed -i -> options writes changes to file

sed -i -e '2d' file.txt -> deletes specific line from file , in this example line 2

sed -i -e '2d;5,10d' file.txt -> deletes lines 2 and 5 through 10

ps aux --sort=-%mem -> sorts the mem column of sort

emergency reset

echo b > /proc/sysrq-trigger

User and Group management

chmod -> change permissions for directories and files

chgrp -> change group owner of file

chage -> change password age of users password

chown -> change user and group ownership of files

id -> get UID, GID , and groups of users via their username

groupmod -> change GID of group

groupmems -> view users apart of a specific group

groupmems -g dudes -l -> view members of a specific group

id -u <username> -> just UID

id -u <username> -> just GID

/etc/default/useradd -> defaults for users
/etc/login.defs -> defaults for users

usermod

usermod -aG wheel <user> -> adds user to wheel group to make them admin privs

useradd

useradd -m -u 1201 -G sales,ops linda -> creates user with home dir and UID of 1201 , adds user to groups sales & ops

sudo file

<username> ALL=/usr/sbin/useradd, /usr/sbin/passwd -> give user the ability to execute useradd and passwd commands without requiring root privs. User still needs to prepend ‘sudo’ before running command

Defaults timestamp_timeout=240 -> sets sudo token timeout

ALL=/usr/bin/useradd, /usr/bin/passwd, ! /usr/bin/passwd root -> prevents users granted passwd execution from changing the password of the root user

sudo sh -c "<command> | <command> -> syntax for piping sudo commands

pkexec visudo -> allows changing the visudo file if access is unavailable

vipw -> allows changing config files for users directly in /etc/passwd and /etc/shadow

vigr -> same as vipw but for changing configs for groups

chage & passwd

passwd -n 30 -w 3 -x 90 linda -> sets minimal password usage to 30 days , with 3 days of warning , expires after 90 days

chage -m 30 -W 3 -M 90 linda -> same thing but using chage instead of apasswd

chage -E 2025-12-31 bob -> sets bobs account to expire at a certain date chage -l -> view current password management settings

Special permissions basics

SUID -> on files , files are executed with the permissions of of the file owner

SGID -> on files , files are executed with permissions of group owner on directories , newly created files are assigned group owner

Sticky bit -> prevents users from deleting files from other users

chmod

4 -> read

2 -> write

1 -> execute

s -> on directories , SGID and execute are set , on files , SUID and execute are set

S -> on directories,only SGID is set , on files , only SUID is set

t -> sticky bit and execute are set

T -> only sticky bit is set

chmod 755 file.txt -> user has full rwx , group has rx , others has rx

chmod -R a+X /home/somedir -> sets execute perm for all users and sub dirs contained within

chmod 4755 /somedir -> set SUID on dir

chmod u+s /somedir

chmod 2755 /somdir -> set SGID on dir

chmod g+s /somedir

chmod 1755 /somedir -> set sticky

chmod +t /somedir

groupmems

groupmems -l -g workers -> lists users in workers group

umask

Subtract perm values

Umask changes default permissions for all users

umask 777 somedir -> defaults for directories

umask 666 somefile -> defaults for files

/etc/profile.d/umask.sh -> file for setting default umask permissions for all users that login to the server , root user created

You could also change the default umask by changing /etc/profile

For individual users you could also add a .profile file in the /etc/skel directory and change the umask there

User-extended attributes

Package: e2fsprogs

A -> file access time , makes sure it’s not modified

a -> allows file to be added but not removed

c -> if volume-level compression is supported, assures files are compressed when the compression engine starts

D -> changes to files are written to disk immediately and not cached first, important for databases

d -> makes sure the file is not backed up when the dump utility is used

I -> enables indexing for the directory where it’s enabled

i -> makes file immutable , good for security

s -> overwrites blocks where file was stored with 0’s after file is deleted , makes sure recovery of the file is not possible after deletion.

u -> saves undelete info , allows a utility that can work with that info to salvage deleted files

chattr +s somefile -> adds s attribute to file

chattr -s somefile -> removes s attribute from file

Review notes for RHCSA

unrelated helpful snippets

output redirection & piping

redirection Examples

chmod

history

umask

Find

Variables

Bash environment

man

mount & mount related commands

ls

tar

gzip & bzip2 & xz

cat & tac

head & tail

cut

sort

wc

grep & regex

awk

sed

emergency reset

User and Group management

usermod

useradd

sudo file

chage & passwd

Special permissions basics

chmod

groupmems

umask

User-extended attributes