Bash

For-loops

#!/bin/bash

for ((i = 0; i < 10; i++)); do
    echo $i
done

Another way to write this is by using the program seq. Seq is pretty much like range() in python. So it can be used like this:

#!/bin/bash

for x in `seq 1 100`; do
    echo $x
done

If statement

$1 here represent the first argument.

if [ "$1" == "" ]; then
    echo "This happens"
fi

If/Else

#!/bin/bash

if [ "$1" == "" ]; then
    echo "This happens"
else
    echo "Something else happens"
fi

Command line arguments

Command line arguments are represented like this

#!/bin/bash

$1

This is the first command line argument

Daemonize an execution

If you do a ping-sweep with host the command will take about a second to complete. And if you run that against 255 hosts I will take a long time to complete. To avoid this we can just deamonize every execution to make it faster. We use the & to daemonize it.

#!/bin/bash

for ip in $(cat ips.txt); do
    ping -c 1 $ip &
done

Use the output of command

It has happened to me several times that I want to input the output of a command into a new command, for example:

I search for a file, find three, and take the last line, which is a path. Now I want to cat that path:

#!/bin/bash

locate 646.c | tail -n 1

This can be done like this:

#!/bin/bash

cat $(locate 646.c | tail -n 1)

Iterate over a file

This script will iterate over a file and echo out every single line

#!/bin/bash

for line in $(cat file.txt);do
    echo $line
done

Another way of writing is this

#!/bin/bash

while read p; do
    echo  $p
done <file.txt

picoCTF flag save

#Create script with below
#File save.sh and chmod +x
#!/bin/bash
original_directory=$(pwd)
cd ..
mv $original_directory ${original_directory}_COMPLETED

#run as follows
source script.sh

#save flag script
#!/bin/bash
winning_command=$(history | tail -n 1 | cut -b 8-)

#Create a new script within this script to pass the value of winning_command
#EOF > End of file
cat <<EOF>./get_flag.sh
#!/bin/bash
${winning_command}
EOF

chmod +x get_flag.sh
get_flag.sh > flag.txt

Running the above must be done after obtaining the flag
run the command .\run | grep -oE "picoCTF{.*}" --color=none
Run only the save.sh
Now that would have created a get_flag.sh and a flag.txt
you can cat the txt file for the flag

Resources

How to Create and Run Bash Shell Script [Beginner's Guide]