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File Maintenance Commands:
Soft link vs Hard link:
- Inode:
- Soft link:
- Hard link:
- This means:
- Example:
Redirecting:
- Standard redirecting:
- Using redirect in combination with EOF:
Pipes (|):
Getting help:
File editors:
User account management:
- commands:
- files:
Switch users and sudo access:
- Commands:
Log monitoring:

— 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

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