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--- //[[mcamav@gmail.com|Marc Verhaar]] 2021/11/02 19:08//
  
{{ :marc:linux:redhat.png?nolink&150|}} ====== Uitreksel RHCSA 8 ======

===== File Maintenance Commands: =====

===== Soft link vs Hard link: =====

==== Inode: ====
  * index position which points to data (file/directory) on drive\\ 
  * Every file has an inode (Index node)
  * Contains all file information **except** file contents and name:
     * Inode number
     * File size
     * Owner information
     * Permissions
     * File type
     * Number of links
     * etc

==== Soft link: ====

* Same as windows shortcut
  * Can span over disks/partitions/lvms
  * Aka Symbolic link
  * Different inode number
  * Smaller file size

==== Hard link: ====

* Different name of the same file
  * Same file size
  * Same inode number
  * Can not span over disks/partitions/lvms (i.e. hard links only work within same partition)

==== This means: ====

* removing the target will render the soft link useless (cat softlink will give an error)
  * removing the target will **not** remove a hard link (cat hardlink will show content of original file)

==== Example: ====
''''
touch original\\ 
echo "some stuff" > original\\ 
ln -s original softlink\\ 
ln original hardlink\\

ls -li\\
**46271969** -rw-rw-r-- **2** marcv marcv **24** Nov  2 20:54 hardlink\\ 
**46271969** -rw-rw-r-- **2** marcv marcv **24** Nov  2 20:54 original\\ 
**46271970** lrwxrwxrwx **1** marcv marcv **8** Nov  2 20:54 softlink -> original\\

rm original\\ 
ls -l\\ 
-rw-rw-r-- **1** marcv marcv 24 Nov  2 20:43 hardlink\\ 
lrwxrwxrwx **1** marcv marcv 8 Nov  2 20:44 ''softlink -> original (broken link)''\\ 
''''

===== Redirecting: =====

==== Standard redirecting: ====

There are 3 redirects in Linux:
  - **stdin**: standard input has file descriptor number as 0 (keyboard, mouse, ..?)
  - **stdout**: standard output has file descriptor number as 1 (monitor)
  - **stderr**: standard error has file descriptor number as 2 (monitor)

We can redirect these output using **>**, **<**, and **2>**:

==== Using redirect in combination with EOF: ====

=== Assign multi-line string to a shell variable: ===

The $sql variable now holds the new-line characters too. You can verify with echo -e "$sql"

=== Pass multi-line string to a file in Bash: ===
<code>cat <<EOF > print.sh
#!/bin/bash
echo \$PWD
echo $PWD
EOF</code>

**The double "<<" is necessary! It does not matter if you use EOF, eof or any other string for closing**

The print.sh file now contains:
<code>#!/bin/bash
echo $PWD
echo /home/user</code>

=== Pass multi-line string to a pipe in Bash: ===
<code>cat <<EOF | grep 'b' | tee b.txt
foo
bar
baz
EOF</code>

The b.txt file contains bar and baz lines. The same output is printed to stdout.

===== Pipes (|): =====

* A pipe is used by the shell to connect the output of one command directly to the input of another command.
  * The symbol is the vertical bar (|)
  * Syntax: command1 [arguments] | command2 [arguments] | command3 [arguments] etc

^ command ^ what it does ^
| <code>ls -ltr | less</code> | feed less with output of ls -ltr |
| <code> find . *file | grep name</code> | find files and grep in filenames(!) |
| <code> cat file* | grep string</code> | cat files and grep inside content(!) This can also be done using <code>grep -R string ./*</code> |

===== Getting help: =====

* whatis command
  * command --help
  * man command
  * help command

===== File editors: =====

* vi
  * ed
  * ex
  * emacs
  * pico
  * vim

For RHCSA you'll be using vi because it's present on almost all *nix systems.

Common keys in command mode:

* i for insert
  * a for append (A for append at end of line)
  * o for new line edit mode
  * r for replace
  * y for yank (copy)
  * d for delete / cut
  * p for paste
  * q for quit

* h for left
  * j for down
  * k for up
  * l for right

* dd: cut line
  * dw: cut word
  * 30dd: cut 30 lines
  * u: undo
  * 3dw: cut 3 words
  * x: cut character
  * 3x: ?

* r-character: replace current character with new

===== User account management: =====

==== commands: ====

* useradd: add user
  * groupadd: add group
  * userdel: delete user
  * groupdel: delete group
  * usermod: modify user
  * chage: change password change requirements (per user)

==== files: ====

* /etc/passwd: contains user information
  * /etc/group: contains group information
  * /etc/shadow: stores actual password in encrypted format and password age (chage)
  * /etc/login.defs: contains system-wide default password policy, also contains UID policy, umask and other stuff

Some examples:

===== Switch users and sudo access: =====

==== Commands: ====
  * sudo su - username: switch to username
  * sudo command: execute command as root
  * visudo: edit /etc/sudoers which defines what users can elevate which privileges

===== Log monitoring: =====

Log directory is /var/log/ (unless specified other per application)

**Accurate system time is critical!**

RedHat 8 uses //[[marc:linux:ntp|NTP]]// for time synchronization.

* boot.log (overwritten on each (re)boot)
  * chrony (NTP)
  * cron
  * maillog (sendmail log)
  * secure: records login attempts (= on RedHat, auth.log on Debian)
  * messages (no 1 for troublesshooting, syslog on Debian)
  * httpd
  * dmesg (on hardware, also to be viewed using command ''dmesg'')
  * firewalld
  * etc

===== Root privileges: =====

Become root using:
<code>su -</code>

The minus ensures that correct environment (variables) is loaded so don't use ''su'' without it.
If configured properly (sudo is installed and user is member of group //wheel//), you can also do:
<code>sudo su -</code>

===== Setting up a rhel server: =====
   * 10Gb root partition
   * 1Gb swap
   * 4Gb unused space

===== CLI commands: =====
  * pwd
  * whoami
  * ls (-l)
  * ip addr show
  * free (-m)
  * df (-h)
  * cat //file//
  * findmnt

===== Understanding the shell: =====
  * tab completion
  * history
  * piping
  * redirectorion
  * environment variables: ''env | less'' will show you all environment variables
  * aliases: ''alias upgrade='sudo apt update && sudo apt upgrade -y'''

01-02-03

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