RH124 Commands to Know

Overview of syslog priorities

0 emerg: system is unusable 1 alert: action must be taken immediately 2 crit: critical condition 3 err: non-critical error condition 4 warning: warning condition 5 notice: normal but sufficent event 6 info: informational event 7 debug: dubgging-level message

Overview of syslog facilities

0 kern: kernel messages 1 user: user-level messages 2 mail: mail system messages 3 dameon: system dameons messages 4 auth: authentication and security messages 5 syslog: internal syslog messages 6 lpr: printer messages 7 news: network news messages 8 uucp: uucp protocol messages 9 cron: clock dameon messages 10 authpriv: non-system authorization messages 11 ftp: ftp protocl messages 16-23 local0 to local7: custom local messages

signal

ANSI C signal handling

echo \$HOME

An escape character in the bash shell. Protects against the following character from expansion

systemctl

Control the systemd system and service manager

groupadd

Create a new group

ln -s

Creates a symbolic link

groupdel

Delete a group

su -

Enter root shell

sudo

Execute a command as another user

- is a regular file d is a directory l is a symbolic link c is a character device file b is a block device p is a named pipe file s is a local socket file

First character of the long listing is the file type

Nine characters that represent file permissions

First three apply to file owner second three apply to file's group owner last set applies to all other users

evince

GNOME document viewer

killall

Kill processes by name

newgrp

Log into a new group.

ln

Make links between files (hard links)

usermod

Modify user account

/var/log/messages

Most syslog messages are logged here

/var/log/boot.log

Non-syslog console messages about system startup

ssh_config

OpenSSH client configuration files

*b*

Pattern matches all files that contain a "b"?

[[:upper:]]*

Pattern matches only file names that begin with an uppercase letter?

???

Pattern matches only file names with at least three characters?

*[[:digit:]]*

Pattern matches only file that contain a number?

b*

Pattern matches only files that begin with "b"?

*b

Pattern that matches file names that end with "b"?

[!b]*

Pattern that matches only the file names where the first character is not "b"?

whoami

Print effective userid

2> file

Redirect stderr to overwrite a file

>> file

Redirect stdout to append to a file

> file

Redirect stdout to overwrite a file

echo '$(command)'

Single quotation marks protect all shell expansion

/var/log/maillog

Syslog messages about mail server

/var/log/cron

Syslog messages about scheduled job execution

/var/logs/secure

Syslog messages about security and authentication events

uptime

Tell how long the system has been running

SSH (Secure Shell)

The most common way to get a shell prompt on a remote system is using Secure Shell. The ssh command encrypts the connection to secure the communication.

passwd

Update user's authentication tokens

/etc/vimrc and ~/.vimrc

Vim configuration file

Changing persmissions with symbolic method chmod Who/What/Which file|directory

Who u - user g - group o - other a - all What + - add - - remove = - set exactly Which r - read w - write x - execute X - special execute

vi/vim

a text file editor in linux

ssh-add

adds private key identities to an authentication agent

man

an interface to the on-line reference manuals

ssh-agent

authentication agent

ssh-keygen

authentication key generation

cd

change directory

touch

change file timestamps. Can be used to create empty files

chage

change user password expiry information

(vim) :q!

command quits vim, and discards all file changes since the last write

(vim) :w

command writes the file and remains in command mode for more editing

cat

concatenate files and print on the standard output

timedatectl

control the system time and date

cp

copy a file or directory

sleep

delay for a specified amount of time

(vim) x

deletes a single character

2> /dev/null

discard stderr error messages by redirecting to /dev/null

echo

display a line of text

pstree

display a tree of processes

echo ${VARIABLENAME}

display a variable in stdout

top

display all running processes

lscpu

display information about the CPU architecture

echo $(command 1)

display the output of a command in stdout

history

displays a list of previously executed commands prefixed with a command number

date

displays the current date and time. +%R shows time and +%x shows the date.

Network interface names

ethernet interfaces begin with "en" WLAN interfaces begin with "wl" WWAN interfaces begin with "ww"

r (read)

file and contents can be read

w (write)

file contents can be changed

x (execute)

files can be executed as commands

VARIABLENAME=value

format for declaring a new variable in Bash

systemd-journald

hear of OS event logging architecture. Collects event messages from many sources: system kernel, output from the early stages of the boot process, stdout and stderr from dameons, syslog events

(vim) u

key undoes the most recent edit

ls

list directory contents

pgrep, pkill

look up or signal processes based on name and other attributes

mkdir

make directory

mv

move a file to another location

head

output the first part of files

tail

output the last part of files

command1 | command2

pipe the output of command1 to the input of command2

wc

print newline, word, and byte counts for each file

id

print real and effective user and group IDs

pwd

print working directory

PID

process id

tee

read from standard input and write to standard output and files

>> file 2>&1 and &>> file

redirect stdout and stderr to append to the same file

> file 2>&1 and &> file

redirect stdout and stderr to overwrite to the same file

rm

remove file or directory

ps

report a snapshot of the current processes

df

report file system disk space usage

su - username

run a command with substitute user and group ID

file

scans the compiled header of a file for a 2-digit magic number and displays its type

find

search for files in a directory hierarchy

rsyslog

service reads syslog messages that the systemd-journald service receives from the journal as they arrive.

w

show who is logged on and what they are doing

kill

terminate a process

Effects of special permissions on files

u+s (suid) - file executes as the user that owns the file, not as the user than ran the file g+s (sgid) - file executes as the group that owns the file o+t (sticky) - no effect

Effects of special permissions on directories

u+s (suid) - no effect g+s (sgid) - files that are created in the directory have a group owner to match the group owner of the directory o+t (sticky) - users with write access to the directory can remove only files that they own; they cannot remove or force save to files that other users own

ssh-copy-id

use locally available keys to authorize logins on a remote machine

(vim) p

vim "put" command aka paste

(vim) y

vim "yank" command aka copy

(vim) :wq

write and quit the file editor