Bash Coding

# Below is all the code to do certain things
# Note that placeholders are used here and this is a guide! Do NOT use this as a script (it will not work)!
# Note also that this is a guide for Bash, so ONLY for Linux (maybe also for MacOS, also based on GNU like Linux)

# Basics
## Restart system
reboot
## Login as user
su userName
### Run code as other user
sudo -u userName code_to_execute
## Remove file/directory
rm file
rm directory
### If the directory not empty
rm -r directory
## Copy
cp fromHere to
### Copy directory with their subdirectorys and files
cp -r fromHere to
## Download files, archives, binarys etc
wget url/file.name
wget url/archive.name
## Run code as process
code_to_execute &

# Advanced
## Declare variables
variable=example
### Declare variables with code to be executed
variable=$(code_to_execute)
## Fetch variable values
$variable
### If you want print a value of a variable in a file
${variable}
### For terminal declared variables (rather known as arguments)
$1
$2
$3

## Dedect OS
uname
## Dedect kernel
uname -r
## Dedect hostname
uname -n
## Dedect Processor
uname -p
## Dedect Architecture
uname -m

## Math
### Greater than
if [ 10 -gt 5 ]; then
    echo "The number is greater than 5"
fi

### Smaller than
if [ 5 -lt 7 ]; then
    echo "The number is less than 7"
fi

### Equal
if [ 5 -eq 5 ]; then
    echo "The number is equal to 5"
fi

## Check for permissions and ownerships
ls -lha

## Fetch configured timezone of the system
cat /etc/timezone

## Write paragraph content in a file
cat << EOF > file.txt
## Content
EOF

## Checks the existence of a PID
if ( kill -0 PID ); then
    # Code to execute
fi

## Checks for the existence of a specific file
if [ -f fileName ]; then
    # Code to execute
fi

## Checks for the existence of a specific directory
if [ -d directory ]; then
    # Code to execute
fi

## Checks, if a directory is writable
if [ -w directory]; then
    # Code to execute
fi

## Checks, if a file is executeable
if [ -x file]; then
    # Code to execute
fi

### You can use this to merge in a one line
[ ! -d directory ] && mkdir directory

### To check whether a directory is empty
if [[ -z "$(find /pfad/zum/verzeichnis -mindepth 1)" ]]; then
    echo "The directory is empty"
else
    echo "The directory is not empty"
fi

## Use functions (handy when you want to use a whole chain of code multiple times, or simply for the overview)
### Create functions
myfunction() {
    # Code_to_execute
}

### Call functions
myfunction

## Give the id of the latest runned process
echo $!
### Save the id of the latest runned process
echo $! > process.pid

## Replace PID
journalctl _PID=PID

## Get all user given arguments
echo $@
### Fetch, if a package exists
checkPackages() {
    for cmd in "$@" ; do
        if command -v "$cmd" ; then
            $cmd --version
            return
        fi
    done

    echo "Not found!"
}

checkPackages thePackage

## Deleting pushed commits in git
### Starting interactive mode
git rebase -i
### Delete commit locally
git reset HEAD~
### Delete commit globally (in the repository)
git push origin main --force --tags

# Experts
## Return the first executing process
ps -p 1 -o comm=

### Check, is used systemd or SysV-Init
if [ "$(ps -p 1 -o comm=)" == "systemd" ]; then
    systemctl enable app
    systemctl start app
else
    service app enable
    service app start
fi

## Set memory swap
fallocate -l 4G /swapfile
chmod 600 /swapfile
mkswap /swapfile
swapon /swapfile

## Let's print "!" 10 times
c=1
while [[ "$c" -le 10 ]]; do
    echo -n "!"
    sleep 1
    c=$(($c+1))
done

## Using getops
while getopts ":a:b:c:d:" opt; do
  case ${opt} in
    a)
      a=$OPTARG
    ;;

    b)
      b=$OPTARG
    ;;

    c)
      c=$OPTARG
    ;;

    d)
      d=$OPTARG
    ;;

    *)
      echo "Option -$OPTARG requires an argument." 1>&2
      exit 1
    ;;
  esac
done

shift $((OPTIND -1))

echo $a
echo $b
echo $c
echo $d

## Using while with shift
while (( "$#" )); do
  case "$1" in
    -h | --host)
      host="$2"
      shift 2
      ;;
    -p | --port)
      port="$2"
      shift 2
      ;;
    -u | --username)
      username="$2"
      shift 2
      ;;
    -pass | --password)
      password="$2"
      shift 2
      ;;
    *)
      echo "Unknown parameter passed: $1"
      exit 1
  esac
done

echo "Host: $host"
echo "Port: $port"
echo "Username: $username"
echo "Password: $password"

## If this error comes:
## Unable to monitor directories for changes because iNotify max watches exceeded, add this to the end of the /etc/sysctl.conf file:
fs.inotify.max_user_watches=99999
### Then run:
sudo sysctl -p
cat /proc/sys/fs/inotify/max_user_watches

## Generate random strings (also usable as passwords)
tr -dc A-Za-z0-9_ < /dev/urandom | head -c 32 | xargs

## Display the IP of the mashine (or computer)
ip addr show | grep -oP '(?<=inet\s)\d+(\.\d+){3}' | grep -v '127.0.0.1' | head -1

## Delete everything in the file file.txt after a :
grep -E ".*:.*" file.txt | sed -i 's/:[^:]*//g' file.txt

## Replace test with test2 in example.txt
sed -i "s|test|test2|" example.txt

## Check for memory (in KiloByte)
awk '/MemTotal/ {print $2}' /proc/meminfo

## Format your echo texts coloured
echo -e "\e[31mThis text is red\e[0m"

### All colors
#### Black: \e[30m
#### Red: \e[31m
#### Green: \e[32m
#### Yellow: \e[33m
#### Blue: \e[34m
#### Magenta: \e[35m
#### Cyan: \e[36m
#### White: \e[37m

### Or
getColoured() {
    tput setaf 1
    echo "$@"
    tput sgr0
}

getColoured test

#### All colors (Put after "tput setaf")
##### Light red: 1
##### Green: 2
##### Yellow: 3
##### Blue: 4
##### Purple: 5
##### Light blue: 6
##### White: 7
##### Grey: 8
##### Red: 9

## Running code based on selected option
options=(
    "Option 1"
    "Option 2"
#   ...
)

select opt in "${options[@]}"
do
    case $opt in
        "Option 1")
            # Code for option 1
        ;;

        "Option 2")
            # Code for option 2
        ;;

#       ...
    esac
done