Shell cheatsheet

Notes from http://www.learnshell.org/
---------------------------------------------

Interpreter name -
   ps | grep $$
   (assumed to be "bash")
  
Interpreter path -
   which bash
   
Shim -
   #!/bin/bash

Variables -
   $VAR_NAME
   #{VAR_NAME}

Capture command output -
   `echo 'command'`
   #(echo 'command')

Arrays -
   arr=(item item2 "item3")
   arr[6]="asd"
   ${arr[6]} # => "asd"
   ${#arr[@]} # => array length
   ${arr[${$arr[@]}]-1} # => last item

Operators -
   $((expression))
   add, substract, multiply, divide, modulo, exponent
   + - * / % **

Numeric Comparison
   Expression | Condition
   -------------------------------------
   $a -lt $b    $a < $b
   $a -gt $b    $a > $b
   $a -le $b    $a <= $b
   $a -ge $b    $a >= $b
   $a -eq $b    $a is equal to $b
   $a -ne $b    $a is not equal to $b

Strings -
   STR="strr"
   ${#STR} # => length
   #{STR:2} # => substring for 2..-1 range
   ${STR:0:2} # => 2-char substring from 0 index
   expr index "asd" "sd" # => first index of substring
   ${STRING[@]// not/} # => remove substring
   ${STRING[@]/be/eat} # => find / replace first
   ${STRING[@]//be/eat} # => find / replace all
   ${STRING[@]/#to be/eat now} # => find / replace substring
   ${STRING[@]/%be/eat} # => find / replace substr if at end
   
String Comparison
   Expression | Condition
   -------------------------------------
   "$a" = "$b"     $a is the same as $b
   "$a" == "$b"    $a is the same as $b
   "$a" != "$b"    $a is different from $b
   -z "$a"         $a is empty

If / Else
   if [ expression ]; then
      echo "true"
   elif [ expression2 ]; then
      echo "also true"
   else
      echo "false"  
   fi

Case
   case "$variable" in
       "$condition1" )
        command...
    ;;
    "$condition2" )
        command...
    ;;
   esac
   
Loop over array
   NAMES=(Joe Jenny Sara Tony)
   for N in ${NAMES[@]} ; do
      echo "My name is $N"
   done

Loop over command results
   for f in $( ls prog.sh /etc/localtime ) ; do
      echo "File is: $f"
   done

Other loop types
   while
   until

Loop keywords
   "break" and "continue"

Functions
   function_name {
      echo "(($1 + $2))" # => arguments
   }
   function_name 1 2 # => call function