Verhaar_Wiki

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— Marc Verhaar 2021/11/02 19:08

====== Uitreksel RHCSA 8 ======

File Maintenance Commands:

Command	Function	Example
cp	Copy file	`cp file1 file2`
rm	Remove file	`rm file2`
mv	Move file (also rename!)	`mv file /path/newfile`
mkdir	Make directory	`mkdir newdir`
rmdir	Remove directory (must be empty!)	`rm newdir`
rm -rf	Remove file or directory	`rm -rf /bin`
chgrp	Change group of file or directory	'' sudo chgrp root file 1''
chown	Change ownership of file or directory	`chown user file`
		`chown user:group file`
		`chown -R user: dir`

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)

Command	Function	Example
ln	create a hardlink	ln target linkname
ln -s	create a soft link	ln -s target linkname
ls -i	show files with inode numbers	ls -li

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>:

Command	Redirects
`ls -l > output`	output to “output” (not overrides output to stdout which is terminal)
`ls -l » output`	same but appends to existing content instead of overwriting
'' cat < file''	redirect content of file to cat (quite useless as cat file will do the same)
`mail user@here.nl < content`	using mailprogram, send content
`ls -l /root 2> errorfile`	redirect errors to errorfile (valid output to stdout)

Using redirect in combination with EOF:

Assign multi-line string to a shell variable:

sql=$(cat <<EOF
SELECT foo, bar FROM db
WHERE foo='baz'
EOF
)

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:

cat <<EOF > print.sh
#!/bin/bash
echo \$PWD
echo $PWD
EOF

The print.sh file now contains:

#!/bin/bash
echo $PWD
echo /home/user

Pass multi-line string to a pipe in Bash:

cat <<EOF | grep 'b' | tee b.txt
foo
bar
baz
EOF

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
ls -ltr \| less	feed less with output of ls -ltr
find . *file \| grep name	find files and grep in filenames(!)
cat file* \| grep string	cat files and grep inside content(!) This can also be done using grep -R string ./*

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:

Command	What it does
useradd -d /homedir -g group username	add user, assign to group and define homedir
usermod -a -G wheel user	add user to additional group wheel
useradd -r nonuser	create system account
useradd -s /bin/zsh user	create user and define shell
chage -m 5 user	user can change password in 5 days
chage -M 90 -W 70 user	user must change password every 90 days and gets warning 7 days prior
chage -I 7 user	account will be disabled 7 days after password expires

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 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 privilegs:

Become root using:

su -

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:

sudo su -

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