variable_argument.md

variable, argument

$var : simplified ${var}
var= var='' : empty variable

• [[ -z $var ]] = true
• [[ -z ${var+x} ]] = false (true if var was not set)

command a b c d e

$# : number of argument = 5
$@ : all arguments = a b c d e
$0 : command itself = command
$1 : Nth argument = a
${@:$#} : last argument = e
${@:(-2):1} : 2nd last = d

$? : last exit code

$* $@ "$@" : all arguments as separated words
"$*" : all arguments as a single word

"${array[@]}" : all value as separated words
"${array[*]}" : all arguments as a single word

eval {...$var} : var inside culrly braces

eval $arg=$var : assign argument as variable name

heredoc to avoid escape

read -r -d '' var <<'EOF'
any characters, quotes, slashs, symbols
can be used without escape
EOF

variable value as variable name

a=b
declare $a=c  # b=c : use value of $a as new variable name
echo $b       # c

indirect variable

a=b
c=a
echo $c      # a
echo ${!c}   # b - expand ${!c} as $a

previous command with arguments

trap 'previouscmd=$thiscmd; thiscmd=$BASH_COMMAND' DEBUG
command arg1 arg2
prev=$previouscmd   # prev='command arg1 arg2'