Command Line Arguments/Substitutions & Referencing Variables

List of some of command line arguments

$0 - Name of the script from the command line

$1 - First command-line argument

$2 - Second command-line argument

$3 - Third command-line argument

$4 - Fourth command-line argument

$5 - Fifth command-line argument

$6 - Sixth command-line argument

$7 - Seventh command-line argument

$8 - Eighth command-line argument

$9 - Ninth command-line argument

$# - Number of command-line arguments

$*, $@ - All command-line arguments, separated with spaces
$? - Return status of previous command
$$ - PID of the current shell under which they are executing

Referencing Variables

a) ${var := word) - If variable value is not set or null then var is set to the value of record

b) ${var :? message) - If variable value is not set or null then Message is printed to standard error

c) ${var:-word} - If var is null or unset, word is substituted for var. The value of var does not change.

d) ${var:+word} - If var is set, word is substituted for var. The value of var does not change.

Part II

# Set the initial value.

myvar=abc

echo “Test 1 ======”

echo $myvar                    # abc

echo ${myvar}                  # same as above, abc

echo {$myvar}                  # {abc}

echo “Test 2 ======”

echo myvar                       # Just the text myvar

echo “myvar”                     # Just the text myvar

echo “$myvar”                   # abc

echo “\$myvar”                  # $myvar

echo “Test 3 ======”

echo $myvardef                # Empty line

echo ${myvar}def             # abcdef

echo ${myvar}{def}           # abc{def}

echo “Test 4 ======”

echo $myvar$myvar         # abcabc

echo ${myvar}${myvar}    # abcabc

echo “Test 5 ======”

# Reset variable value, with spaces

myvar=”a b c”

echo “$myvar”                  # a b c

echo $myvar                    # a b c

echo “Test 6 ======”

# Difference between single quotes and double quotes

myvar=abc

echo "$myvar"                 # abc    

echo '$myvar'                  # $myvar

echo " '$myvar' "             #  'abc'        ; ' ' has no special meaning inside " "
echo ' "$myvar" '             # "$myvar"  ;  " " is treated literally inside ' '