Linux

Useful commands quick reference

Administration

Users

openssl passwd <password>
useradd -p "<password hash>" <username>

Add by editing /etc/passwd

echo "<username>:$(openssl passwd <password>):0:0:/root:/root:/bin/bash" >> /etc/passwd

Groups

id
groups
getent group

Add group

sudo groupadd <group name>

Assign user to group

usermod -a -G <group> <user>
usermod -g <group> <user>    #transfer user to group

usermod -a -G root <user>    #add to root
usermod -a -G rdp <user>     #add to rdp enabled group
usermod -a -G ftp <user>     #add to ftp enabled group

Firewall

iptables -L
iptables -S
iptables --flush

Directories and files

List subdirs

find . -type d 2>/dev/null
find / -type d -readable -exec ls -adl {} \; 2>/dev/null

List files

find . -type f -name "<name>" 2>/dev/null
find . -type f -name "*.conf" 2>/dev/null
find . -type f -name "*.txt"  2>/dev/null
find . -type f -mtime +<days> 2>/dev/null                   #files older than x days
find . -type f -name"<name>" -exec cat {} \; 2>/dev/null    #find and print file
ls -h  
ls -lah

Mounting

mount / unmount

mount    #list currently mounted resources
mount -t <type> <ip>:/<share path> <path to mount point>
umount <path to mount point>
umount -f <path>

Resolve "file is busy" unmount error

fuser <path to mount point>     #list all processes accessing the mount point
fuser -k <path to mount>        #kill all processes accessing the mount point

Permissions

Permission types

Octal	Decimal	Representation	Permission
000	0	---	No permission
001	1	--x	Execute
010	2	-w-	Write
011	3	-wx	Write+Execute
100	4	r--	Read
101	5	r--x	Read+Execute
110	6	rw-	Read+Write
111	7	rwx	Read+Write+Execute

Check permissions

ls -alh <file>

Alter permissions

chmod +<repr> <file> #add permission
chmod -<repr> <file> #drop permission
chmod <decimal for user,group,others i.e. 760> <file> #set all for users, r+w for owner group, none for others

chown <user> <file>            #change file owner
chown <user>:<group> <file>    #change file owner specifying user's group

Archives

Tar

--Add to archive--
tar -cf <archive>.tar <file1 file2... or dir/>
tar -uvf <archive>.tar <file to add>
--List files--
tar -tvf <archive>.tar
--Compress--
tar -czvf archive.tar.gz <dir>/
--Extract--
tar -xvf <archive>.tar -C <path to extract>
tar -xzvf <archive>.tar.gz -C <path to extract>

Zip

--Archive--
7z a -t<type>  <archive>.<type> <dir>
7z a -mhe=on -p<password> <archive>.7z <dir>
--Extract--
7z e <archive>.<type> -o <dir output>
--Extract full paths--
7z x <archive>.<7z,zip,gzip,bzip2,tar> -o <dir output>
gunzip <archive>.zip > <filename>.txt

Text

Extract

Between two delimiters (from pipe or specify filename)

sed -n '/<begin>/,<end>/{/<begin>/b;/<end>/b;p}'

Get string length

echo -n "<str>" | wc -c

Read content from .gz archive

zcat <.gz archive>

Base64

echo "<string>" | base64
echo -n "<base64 string>" | base64 -d
base64 -d <file> > <decoded file>

Hex

xxd -p <file> | tr -d '\n' > <hexfile>.hex
echo -n "<hex string>" | xxd -r -ps
cat <hexfile>.hex | xclip -selection clipboard

Useful Commands

su -c '<command>' <user>    
su - <user> -c '<command>'    #may ask for password
sudo -u <user> <cmd>          #simulate user login


find -type f -name "*.txt" -exec cat "{}" \;
find -type f -name "*.txt" -exec mv "{}" <dst> \;    #useful when mv throws errors
find -exec mv "{}" <dst> \;                          #move all files and folders from here

ls | wc -l    #count files in folder
wc -l <file>  #count the lines in file
wc -w <file>  #count the words in file

Networking

Default TTL

64

Typical Ports

22    SSH
111   RPC
123   NTP
2049  NFS