Shell Scripting Basics

#!/bin/bash (Header .Sh)
echo"text" ("print()")
chmod u+x file.Sh
g++ add.Cpp -o name (compile)
./add < input.Txt > output.Txt
pet="dog" (variable)
echo "Hello $var" (access var)
read var (input())
echo "$x" ("" echo var echo '$x')
echo "\$x" (echo special char)
a="`cat fl.Txt`" (stores shell cmd output)
echo ${#var} (No. Char in var)
echo ${a:6:3} (return substring ${a:pos:len})
echo ${a/$from/$to} (replace 'from' to 'to)
let "b = $a + 9" (does math and ans strd in var)
var="$0" ("$0" cmd line arg, "$@", @:all)

if [ condition ] | [ "$string" ] True; length of $string is non-0 | [ -e $file ] True iff file exists
then | [ "$string1" == "$string2" ] True; strings are equal | [ -f $file ] True iff file is a file
some action | [ "$string1" != "$string2" ] True; strings are different | [ -d $file ] True iff file is a directory
elif | [ "$string1" \> "$string2" ]True; $string1 is sorted after $string2 | [ -s $file ] True iff file has size > 0
then | [ "$string1" \> "$string2" ] True; $string1 is sorted after $string2 | [ -r $file ] True iff file is readable
some action | [ "$string1" \< "$string2" ]True; $string1 is sorted before $string2 | [ -w $file ] True iff file is writable
fi | [ -x $file ] True iff file is executable