intermediate_bash.md

Intermediate Bash

Globs

# list all files ending with either pdf or md
ls {*.pdf,*.md}
# match every file starting with i, then a pattern zero or more times
# followed by an m
ls i?(nter)m*

Networking

nc

Anything TCP, UDP and sockets related. Connects and listens.

# Look at the manual
man nc

Create a connection between your two terminals Terminal 1

# nc -l <port>
nc -l 8743

Terminal 2

# nc <your ip> <port specified in terminal 1>
nc 123.123.123.123 8743
# Write anything and see it shown in Terminal 1

ss

# show listening sockets and guessing the use of the socket
ss -tl
# show listening sockets without guessing the use of the socket
ss -tln

Security Bits

SUID & SGID [EXAMPLE NOT WORKING]

Set user identification (SUID) allow a user to run a program as some other user (like root).

# creating folders there is only allowed by the root user
mkdir '/srv/newfolder'

mkdir 'removabletestfolder'
cd 'removabletestfolder'

# create a program which does it
cat << EOF > 'mytestprogram.sh'
mkdir '/srv/newfolder'
EOF

# 'mytestprogram.sh' file is there, owned by the current user
ls -l

# change ownership of the program to root user and root group
sudo chown root:root 'mytestprogram.sh'

# 'mytestprogram.sh' file is there, owned by the root user
ls -l

sudo chmod u+x,g+x,o+x 'mytestprogram.sh'
# change file mode bits
sudo chmod u+s 'mytestprogram.sh'

# should run :), but currently doesn't
bash 'mytestprogram.sh'

Other Helpful Bits

!$

!$ brings back the last argument of the previous command.

# echo prints all arguments (separated by space)
echo 1 2 3  # output: 1 2 3
echo !$     # output: 3

# even though space separates arguments, it's still inserted
echo 1 "2 3"    # output: 1 2 3
echo !$         # output: 2 3

echo 1 2 3"4 5" # output: 1 2 34 5
echo !$         # output: 34 5

Arithmetic

bc

Basic Calculator

echo "1+1" | bc

let

let "x=1+1"
echo $x