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Difference between ${} and $() in Bash [duplicate]

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What does ${} mean in bash?

${} Parameter Substitution/Expansion A parameter, in Bash, is an entity that is used to store values. A parameter can be referenced by a number, a name, or by a special symbol.

What's the difference between $@ and $* in bash?

There is no difference if you do not put $* or $@ in quotes. But if you put them inside quotes (which you should, as a general good practice), then $@ will pass your parameters as separate parameters, whereas $* will just pass all params as a single parameter.

What are $( and $(( )) in bash?

$(...) is an expression that starts a new subshell, whose expansion is the standard output produced by the commands it runs. This is similar to another command/expression pair in bash : ((...)) is an arithmetic statement, while $((...)) is an arithmetic expression. Follow this answer to receive notifications.

What is difference between $@ and $*?

Difference Between Asterisk ( $* ) and At-Sign ( $@ ) in Bash Scripting. The $* expression starts from one and expands to the positional parameters. If the expression is not used with double quotes, each positional parameter expands to a separate word.


The syntax is token-level, so the meaning of the dollar sign depends on the token it's in. The expression $(command) is a modern synonym for `command` which stands for command substitution; it means run command and put its output here. So

echo "Today is $(date). A fine day."

will run the date command and include its output in the argument to echo. The parentheses are unrelated to the syntax for running a command in a subshell, although they have something in common (the command substitution also runs in a separate subshell).

By contrast, ${variable} is just a disambiguation mechanism, so you can say ${var}text when you mean the contents of the variable var, followed by text (as opposed to $vartext which means the contents of the variable vartext).

The while loop expects a single argument which should evaluate to true or false (or actually multiple, where the last one's truth value is examined -- thanks Jonathan Leffler for pointing this out); when it's false, the loop is no longer executed. The for loop iterates over a list of items and binds each to a loop variable in turn; the syntax you refer to is one (rather generalized) way to express a loop over a range of arithmetic values.

A for loop like that can be rephrased as a while loop. The expression

for ((init; check; step)); do
    body
done

is equivalent to

init
while check; do
    body
    step
done

It makes sense to keep all the loop control in one place for legibility; but as you can see when it's expressed like this, the for loop does quite a bit more than the while loop.

Of course, this syntax is Bash-specific; classic Bourne shell only has

for variable in token1 token2 ...; do

(Somewhat more elegantly, you could avoid the echo in the first example as long as you are sure that your argument string doesn't contain any % format codes:

date +'Today is %c. A fine day.'

Avoiding a process where you can is an important consideration, even though it doesn't make a lot of difference in this isolated example.)


  1. $() means: "first evaluate this, and then evaluate the rest of the line".

    Ex :

    echo $(pwd)/myFile.txt
    

    will be interpreted as

    echo /my/path/myFile.txt
    

    On the other hand ${} expands a variable.

    Ex:

    MY_VAR=toto
    echo ${MY_VAR}/myFile.txt
    

    will be interpreted as

    echo toto/myFile.txt
    
  2. Why can't I use it as bash$ while ((i=0;i<10;i++)); do echo $i; done

    I'm afraid the answer is just that the bash syntax for while just isn't the same as the syntax for for.


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