Bash Scripting

a=hi
echo $a \\(planet\\)    # hi (planet)
echo '$a (planet)'      # $a (planet) 
echo "$a (planet)"      # hi (planet)
echo                    # empty line

a=Hello
b="Hello World"
c=16
echo $a     # Hello
echo $b     # Hello World

d=$(pwd)
echo $d     # /home/user/dir

a=2
b=$((a+2))
echo $b     # 4

[[ "cat" == "cat" ]]
echo $?     # 0 (True)

[[ "cat" = "car" ]]
echo $?     # 1 (False)

[[ 20 -gt 100 ]]
echo $?     # 1 (False)

a=""
b="cat"
[[ -z $a && -n $b ]]
echo $?     # 0 (True)

a="hello"
b="world"
c=$a$b
echo $c         # helloworld
echo ${#c}      # 10
echo ${c:3}     # loworld
echo ${c: -4}   # orld
echo ${c:3:4}   # lowo
echo ${c: -4:3} # orl

fruit="apple banana banana orange"
echo ${fruit/banana/kiwi}        # apple kiwi banana orange
echo ${fruit//banana/kiwi}       # apple kiwi kiwi orange
echo ${fruit/#apple/kiwi}        # it replace only if it is begining of the string
echo ${fruit/%orange/kiwi}       # it replace only if it is end of the string
echo ${fruit/o*/kiwi}            # pattern matching # apple banana banana kiwi

flashred="\\033[5;31;40m"
red="\\033[31;40m"
none="\\033[0m"
echo -e $flashred"Error: "$none$red"Something went wrong"$none

today=$(date +"%d-%m-%Y")
time=$(date +"%H:%M:%S")
printf -v d "Current User:\\t%s\\nTime:\\t\\t%s @ %s\\n" $USER $today $time
echo "$d"

a=() # empty array
b=("apple" "banana" "orange") # three elements in array
echo ${b[2]} # orange
b[5]="kiwi"
b+=("mango") # add element to the end of array
echo ${b[@]} # echo whole array
echo ${b[@]: -1} # echo last element of array
# like map
declare -A myarray
myarray[blue]=color
myarray["two words"]="another words"
echo ${myarray[blue]} and ${myaaray["two words"]}

echo "Some text" > file.txt # write to file.txt - creates file if doesn't exist
> file.txt # zero out file.txt - make it empty
echo "Some more text" >> file.txt # writes to end of file.txt

# to read file.txt
i=1
while read f; do
    echo "Line $i: $f"
    ((i++))
done < file.txt

cat < file.txt # use cat to read the file

# to print some long text or instructions
cat <<- EOF
Some really long
text, like help,
or like documents.
EOF

# to run some commands
ftp -n <<- DownloadDocument
    open mirrors.xmission.com
    user anonymous nopass
    ascii
    cd gutenberg
    get GUTINDEX.00
    bye
DownloadDocument

if expression; then
    echo "True"
elif expression2; then
    echo "ex False, ex2 True"
else
    echo "False"
fi

i=0
while [ $i -le 10 ]; do
    echo i: $i
    ((i+=1))
done
j=0
until [ $j -ge 10 ]; do
    echo j: $j
    ((j+=1))
done

for i in {1..100}
do
    echo $i
done

# another variant
for (( i=1; i<=10; i++ ))
do
    echo $i
done

# array
arr=("apple" "banana" "cherry" "kiwi" "mango")
for i in ${arr[@]}
do
    echo $i
done 

# command substitution
for i in $(ls)
do
    echo $i
done 

a="dog"
case $a in
    cat) echo "miow miow";;
    dog|puppy) echo "huf huf";;
    *) echo "No match";;
esac

greet() {
    echo "Hi $1. What a nice $2"
}
echo "And now greetings."
greet Ben Morning
greet Everybody Evening

numberthings() {
    i=1
    for f in $@; do
        echo $i: $f
        ((i+=1))
    done    
}

numberthings $(ls)
numberthings haskell elm python

echo $1 $2 # first and second argument

for i in $@
    do
        echo $i
    done

echo "There were $# arguments."

while getopts :u:p:ab option; do # : means I do wait some value for u: p:
    case $option in              # for ab I just want to know if it was used
        u) user=$OPTARG;;        # if : is used before then I want check flags which are not specified
        p) pass=$OPTARG;;
        a) echo "got the flag a";;
        b) echo "got the flag b";;
        ?) echo "I don't know what $OPTARG is";;
    esac
done
echo "User: $user / Pass: $pass"

echo "What is your name?"
read name
echo "What is your password?"
read -s pass # silent you will not see characters while typing

read -p "What is your favorite language? " lang
echo name: $name, pass: $pass, lang: $lang 

select animal in "cat" "dog" "spider" "fly"
do
    echo "You selected: $animal"
    break
done

select option in "cat" "dog" "quit"
do
    case $option in
        cat) echo "Cats like cuddle";;
        dog) echo "Dogs like to run";;
        quit) break;;
        *) echo "I am not sure what that is";;
    esac
done

# checks number of arguments, if not enough it will write message
if [[ $# -lt 3 ]]; then
    cat <<- EOF
        This command requires three arguments:
        username, userid and favorit number.
    EOF
else
    echo "Username: $1"
    echo "Userid: $2"
    echo "Favorite number: $3"
fi

# till option is satisfied
read -p "Favorite animal? " a
while [[ -z "$a" ]]; do
    read -p "I need an answer! " a
done
echo "$a was selected."

# predefined option
read -p "Favorite animal? [cat] " a
while [[ -z "$a" ]]; do
    a="cat"
done
echo "$a was selected."

# checking input with regexp
read -p "Which year? [nnnn]" year
while [[ ! $year =~ [0-9]{4} ]]; do
    read -p "A year please! [nnnn]" year
done
echo "Selected year: $year"