Shell symbol magic

Shell languages like bash have some interesting syntax that looks like magic as it is just symbols in the syntax and not a single word actually written. This makes writing these things extremely hard to remember or write but once you know them you can do quite some useful things using shells.

Setting up for testing
Symbolry
End

Setting up for testing

We are going to focus on Bash for this as there is dash, zsh and a bunch of others but what is inside bash is very similar to zsh and works almost identical except a few quirks. Most of these symbol related syntaxes are called expansions or substitutions.

To begin let’s define some variables to use to test these where one is a string, second one a simple array and third is resetting the shell input often called ARGV and is the arguments passed to the current script or function.

var='this is a variable with lowercase and UPPERCASE'
arr=('after ' ' before' ' surrounded ' 'alone')
set 'one ' ' two ' 'three'

Also we need to define how to actually display the output which we will do with this where lines starting with $ are commands being run.

$ printf %q\\n several "arguments here" to test ' this '
several
arguments\ here
to
test
\ this\

As it prints one thing per line escaped we can see what is actually shaped by these symbols. Now on to what ones there are.

Symbolry

Let’s get started with the magic symbols.

”$@” each command line argument as separate argumemnt

$ printf %q\\n "$@"
one\
\ two\
three

Good for getting all shell arguments to pass elsewhere.

$* or $@ all shell args

$ printf %q\\n $*
one
two
three
$ set ' a ' 'b ' ' c d '
a
b
c
d

This does same as above but takes away spaces.

”$*” all shell args into one

$ printf %q\\n "$*"
one\ \ \ two\ \ three

Yes this packs everything into a single argument.

$@ “$@” each shell arg into separate argument

$ printf %q\\n "$@"
one\
\ two\
three

${#var} length of string

$ printf %q\\n "${#var}"
47

Yes the length.

${#arr[@]} length of an array

$ printf %q\\n "${#arr[@]}"
4

Yep the length.

”${arr[@]}” all members of array with each element as an argument

printf %q\\n "${arr[@]}"
after\
\ before
\ surrounded\
alone

”${arr[@]}” all members of array with each element as an argument but losing spaces

$ printf %q\\n ${arr[@]}
after
before
surrounded
alone

”${arr[*]}” all members of array joined by space into a single argument

$ printf %q\\n "${arr[*]}"
after\ \ \ before\ \ surrounded\ \ alone

Good for joining arrays.

${arr[*]} , “${arr[@]}” or ${arr[@]} all members of array but cut members up

$ printf %q\\n ${arr[*]}
after\
\ before
\ surrounded\
alone
$ printf %q\\n ${arr[@]}
after\
\ before
\ surrounded\
alone
$ printf %q\\n "${arr[@]}"
after\
\ before
\ surrounded\
alone

${arr[@]:start} or ${arr[@]:start:length} slice an array

$ printf %q\\n "${arr[@]:2}"
\ surrounded\
alone
$ printf %q\\n "${arr[@]:2:1}"
\ surrounded\
$ printf %q\\n ${arr[@]:1:1}
\ before
$ printf %q\\n ${arr[@]:3}
alone

${var#string} cut off beginning

$ printf %q\\n ${var#}
this\ is\ a\ variable\ with\ lowercase\ and\ UPPERCASE
$ printf %q\\n ${var#var}
this\ is\ a\ variable\ with\ lowercase\ and\ UPPERCASE
$ printf %q\\n ${var#*var}
iable\ with\ lowercase\ and\ UPPERCASE
$ printf %q\\n ${var#this}
\ is\ a\ variable\ with\ lowercase\ and\ UPPERCASE
$ printf %q\\n ${var#}
this\ is\ a\ variable\ with\ lowercase\ and\ UPPERCASE
$ printf %q\\n ${var#*var}
iable\ with\ lowercase\ and\ UPPERCASE

Quite convenient substring like feature.

${var%string} cut off end

$ printf %q\\n ${var%and*}
this\ is\ a\ variable\ with\ lowercase\
$ printf %q\\n ${var%with*}
this\ is\ a\ variable\
$ printf %q\\n ${var%CASE}
this\ is\ a\ variable\ with\ lowercase\ and\ UPPER
$ printf %q\\n ${var%low*}
this\ is\ a\ variable\ with\
$ printf %q\\n ${var% *}
this\ is\ a\ variable\ with\ lowercase\ and

Quite useful but might be hard to figure out

${var-default} get value of variable or return default if empty

$ printf %q\\n ${var-default}
this\ is\ a\ variable\ with\ lowercase\ and\ UPPERCASE
$ printf %q\\n ${nonexistent-default}
default

Good when a variable might not be set.

${var:=default} get value of variable or set default if empty

printf %q\\n ${var:=default value}
this\ is\ a\ variable\ with\ lowercase\ and\ UPPERCASE
$ printf %q\\n ${varr:=default value}
default\ value
$ printf %q\\n ${varr}
default\ value

Similar to Python’s dict.setdefault.

${var:start} or ${var:start:length} substring

$ printf %q\\n ${var:10}
variable\ with\ lowercase\ and\ UPPERCASE
$ printf %q\\n ${var:10:6}
variab
$ printf %q\\n ${var:1:10}
his\ is\ a\ v

Fully functional substring with a length argument that is better than those that want you to specify the end.

${var^^} uppercase string

$ printf %q\\n ${var^^}
THIS\ IS\ A\ VARIABLE\ WITH\ LOWERCASE\ AND\ UPPERCASE

VERY!

${var,,} lowercase string

$ printf %q\\n ${var,,}
this\ is\ a\ variable\ with\ lowercase\ and\ uppercase

no uppercase anymore.

$(( )) do math

$ printf %q\\n $(( 42 - 69 + 2 + 1337 ))
1312

Precisely what it says.

${var[@]@A}

$ echo ${var@A}
var='this is a variable with lowercase and UPPERCASE'
$ echo ${arr[@]@A}
declare -a arr=([0]="after " [1]=" before" [2]=" surrounded " [3]="alone")

Notice the echo here but also yes this gives the array definition or string definition needed to properly export the variable

${var[@]@Q}

$ echo ${var@Q}
'this is a variable with lowercase and UPPERCASE'
$ echo ${arr[*]@Q}
'after ' ' before' ' surrounded ' 'alone'
$ echo ${arr[@]@Q}
'after ' ' before' ' surrounded ' 'alone'

This quotes stuff.

~- ~+ ~ $PWD $OLDPWD $HOME

$ printf %q\\n ~- ~+ ~ $PWD $OLDPWD $HOME
/home
/home/pi
/home/pi
/home/pi
/home
/home/pi

End

This shows that shells do indeed have quite some features that can be used for quite a few tasks and while many of these tricks do not work in zsh it has its own magic.

*Mweeoops*

YeenDeer softness blog (programming and electronics)

Ellie the Yeen is a soft YeenDeer that mweeoops and does programming