Bash

Bash Scripting:

if:

if [ ! -f .env ]; then
    echo hello
elif podman compose version >/dev/null 2>&1; then
    echo podman rocks!!
else
    echo bye
fi

input prompt:

echo "Do you wish to install this program?"
select yn in "Yes" "No"; do
    case $yn in
        Yes ) make install; break;;
        No ) exit;;
    esac
done

Variables:

leave no space in between varaiable and value

a=hello
c="more then one word"
b=3

Arithmatic Expansion

need two paranthesis to evaluate a math function else treated like string

sum=$((3+1))

Datatypes:

string, number, array, associative array (dict)

arrays:

# Array example
fruits=("apple" "banana" "cherry")
for fruit in "${fruits[@]}"; do
  echo $fruit
done

associative array:

# Associative array example
declare -A colors
colors[apple]="red"
colors[banana]="yellow"
colors[grape]="purple"
unset colors[banana]
echo ${colors[apple]} # red
echo ${colors[grape]} # purple

add to path:

export PATH=$PATH:/path/to/dir

cli argument parsing

sum=$((3+1))
good="more then one word"
argument_count=$#

echo "the argument count is: $argument_count"
echo hi
echo $good
echo $sum

if [ "$argument_count" -ne 1 ]; then
    echo "please pass 1 and just 1 input after this file"
else
    echo $1
fi

else set default value:

FIRST_OR_DEFAULT="${FIRST:-default_value_here}"

Loops

for file in *.log; do
  echo "Processing $file"
done

Functions

greet() {
  local name=$1
  echo "Hello, $name!"
}
greet "Alice"

Operators:

Comparison Operators

• -eq: Equal to
• -ne: Not equal to
• -lt: Less than
• -le: Less than or equal to
• -gt: Greater than
• -ge: Greater than or equal to

---

String Comparison Operators

• =: Equal to
• !=: Not equal to
• <: Less than, in ASCII alphabetical order
• >: Greater than, in ASCII alphabetical order

---

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Arithmetic Operators

• +: Addition
• -: Subtraction
• *: Multiplication
• /: Division
• %: Modulus (remainder of division)
• For exponentiation, use external tools like bc or awk.

---

Logical Operators

• &&: Logical AND
• ||: Logical OR
• !: Logical NOT

---

File Test Operators

• -e: Checks if a file exists
• -d: Checks if a directory exists
• -f: Checks if a file is a regular file
• -s: Checks if a file is not empty

source: https://www.w3schools.com/bash/bash_operators.php

---

trap

Commands to use:

Useful Commands:

history | grep apt

tar -czvf /backup/home_$(date +%F).tar.gz /home/user

find /var/log -type f -name "*.log" -mtime +7 -exec gzip {} \;

ping -c 4 8.8.8.8

docker ps -q | xargs -r docker stop
docker system prune -af

while true; do echo "$(date): $(uptime)"; sleep 60; done >> uptime.log

Basic Commands:

Command	Notes	Parameters	Use Frequency
mkdir	can create multiple directories	-p - Create parent directories as needed -v - Show a message for each created directory -m - Set file mode (permissions)	often
rm		-r - Delete a folder and everything inside it -i - Ask before deleting each file -f - Force delete without asking -v - Verbose mode, show files being removed	often
cat		-n - Add numbers to each line -b - Add numbers only to lines with text -s - Remove extra empty lines -v - Show non-printing characters (except for tabs and end of line)	commonly
alias	can create alias and also check what aliases are set		setup once occasionaly
man	read manuals of clis		rarely
touch	can create files but also update timestamps of existing files		rare
ls
pwd
cd
echo
cp
mv
info	more info than man

TAR

tar --zstd -xvf anki-launcher-25.09-linux.tar.zst

Bash Search Text (grep)
Bash Pattern Scan (awk)
Bash Stream Editor (sed)
Bash Remove Section (cut)
Bash Sort Lines (sort)
Bash View End (tail)
Bash View Start (head)

Bash Process Status (ps)
Bash List Processes (top)
Bash Disk Space (df)
Bash Directory Usage (du)
Bash Memory Usage (free)
Bash Terminate (kill)
Bash Uptime (uptime)

Bash Ping (ping)
Bash URL Transfer (curl)
Bash Downloader (wget)
Bash Remote Connect (ssh)
Bash Secure Copy (scp)
Bash File Sync (rsync)

File Compression
Bash Compress (zip)
Bash Extract (unzip)
Bash TAR Archive (tar)

File Permissions
Bash Ownership
Bash Modify (chmod)
Bash Ownership (chown)
Bash Group (chgrp)

Bash Schedule (cron)

crontab

• -e: Edit the crontab file for the current user.
• -l: List the crontab entries for the current user.
• -r: Remove the crontab file for the current user.

Setting Up Cron Jobs

Cron jobs are defined using a specific syntax in the crontab file. Each line in the file represents a task and follows this format:

* * * * * command_to_execute

• Minute: 0-59
• Hour: 0-23
• Day of Month: 1-31
• Month: 1-12
• Day of Week: 0-7 (0 and 7 are Sunday)

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As of Bash 5.3, the 10 metacharacters are the space, tab, and newline, as well as the following characters: |&;()<>

Bash Keyboard shortcuts

• Ctrl+e – Move the cursor to the end of the current commandline
• Ctrl+a – Move the cursor to the beginning of the current commandline
• Ctrl+w – Remove one word to the left of the cursor
• Ctrl+s – Put the terminal to sleep
• Ctrl+q – Wake the terminal; buffered keypresses are then processed
• Ctrl+z – Stop a foregrounded process

https://en.wikipedia.org/wiki/Bash_(Unix_shell)