Braces in bash

Do you know the difference between ${}, $(), $(()) and just $ in shell?

Explanation

$ - just extracts a variable. Example:

$ echo $PWD 
/home/energyc0

${} - also extracts a variable, but with some features. Example:

$ echo ${PWD}
/home/energyc0

Also you can modify variables "on fly". This is called “Parameter Expansion” - https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html and https://tldp.org/LDP/abs/html/parameter-substitution.html. Example:

$ var=abcdef
$ rep='& '
$ echo ${var/abc/& }
abc def
$ echo "${var/abc/& }"
abc def
$ echo ${var/abc/$rep}
abc def
$ echo "${var/abc/$rep}"
abc def

$ A="abababcdef"
$ echo ${A#*ab}
ababcdef
$ echo ${A##*ab}
cdef

$() means: “firstly, evaluate this and then the other”. Example:

$ echo "Today is $(date). Beautiful day!"
Today is Sun Sep  7 08:50:37 AM MSK 2025. Beautiful day!

So you can use output of different commands in shell to run other commands!

$(()) evaluates an expression. Example:

$ echo $((5*2/4 + 1))
3

As you can see, it works with integers.

One problem

Just explore these examples:

var="abc"
if [ $var == abc def ]; then
    echo "Victory!"
else
    echo "Defeat!"
fi

bash: [: too many arguments
Defeat!

var="abc"
if [ $var == "abc def" ]; then
    echo "Victory!"
else
    echo "Defeat!"
fi

Defeat!

var="abc def"
if [ $var == "abc def" ]; then
    echo "Victory!"
else
    echo "Defeat!"
fi

bash: [: too many arguments
Defeat!

var="abc def"
if [ ${var} == "abc def" ]; then
    echo "Victory!"
else
    echo "Defeat!"
fi

bash: [: too many arguments
Defeat!

var="abc def"
if [ "$var" == "abc def" ]; then
    echo "Victory!"
else
    echo "Defeat!"
fi

Victory!

The only conclusion you can make is that you need “” to use variables correctly.