TryHackMe: Pre Security (Supplements)

author: Nathan Acks
date: 2021-09-20

Bash Scripting

Variables

Using the -x flag will force bash to output each line of the shell script you’re running before that line is executed. This can be useful for debugging.

bash -x ./script.sh

The -x flag can also be incorporated into the interpreter line.

#!/usr/bin/env bash -x

# Script content...

Finally, this mode can be toggled on and off with the set command within the script itself.

#!/usr/bin/env bash

# Some script content...

set -x

# These lines will be echoed before execution.

set +x

# These lines will not be echoed...

Frequently set -x is used at the start of a script without a closing set + x, which will just cause all lines of the script to be echoed back before execution.

Parameters

The read command in bash will set the variable name supplied to whatever is provided on STDIN (presumably up to IFS). For example:

#!/usr/bin/env bash

echo -n "Enter something: ""

read INPUT

echo "You typed: $INPUT"

Will echo back whatever is typed in, prefixed by “You typed: “.

Arrays

Bash actually does support arrays.

MY_ARRAY=('item0' 'item1' 'item2')

As the above example suggests, arrays are 0-indexed. Use @ to output the full (space-separated) array.

echo "$MY_ARRAY"      # item0 (weird!)
echo "${MY_ARRAY[@]}" # item0 item1 item2
echo "${MY_ARRAY[1]}" # item1

unset MY_ARRAY[1]
echo "${MY_ARRAY[@]}" # item0 item2

# But be aware that this DOESN't change the index of any
# element!

echo "${MY_ARRAY[0]}" # item0
echo "${MY_ARRAY[1]}" # Outputs nothing...
echo "${MY_ARRAY[2]}" # item2

MY_ARRAY[0]="item3"
echo "${MY_ARRAY[@]}" # item3 item2

MY_ARRAY[3]="item4"
echo "${MY_ARRAY[@]}" # item3 item2 item4