Difference between ${} and $() in Bash

Question

I have two questions and could use some help understanding them.

What is the difference between ${} and $()? I understand that () means running command in separate shell and placing $ means passing the value to variable. Can someone help me in understanding this? Please correct me if I am wrong.
If we can use for ((i=0;i<10;i++)); do echo $i; done and it works fine then why can't I use it as while ((i=0;i<10;i++)); do echo $i; done? What is the difference in execution cycle for both?

It is a 2 part question. "Backticks vs. braces" just happens to have answers for both? If it answers 1, ask OP to edit it so that it is a standard, singular question. — Nate T, Sep 11 '21 at 08:31

score 262 · Accepted Answer · edited Jan 20 '20 at 22:28

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.)

Strictly, a `while` command expects a sequence of commands (not necessarily a pipeline), and the exit status of the last command in the sequence determines whether the loop continues or not. Thus: `while ls; pwd; date; do echo Hi; sleep 1; done` is valid, and the exit status of `date` controls whether the loop continues or not. I used semicolons because you can't format newlines in a comment, but you could write the commands on multiple lines in a shell script. — Jonathan Leffler, Dec 14 '14 at 19:46
I'm flabbergasted that this answer continues to receive upvotes; it's not particularly stellar. If you upvoted this, would you care to explain which part of it is useful? I guess the relationship between `for` loops and `while` loops isn't entirely obvious (I remember liking it when I first read it in on the first edition of the Perl *Camel Book)* but is there anything other than that ...? — tripleee, Apr 09 '18 at 06:40
There's also the issue of 'well, the other two answers are not better'. — Jonathan Leffler, Apr 09 '18 at 07:56
@JonathanLeffler Heh, maybe just abstain from voting then (-: — tripleee, Apr 09 '18 at 08:03
@tripleee Why don't you want the upvotes? Anyway I often upvote answers because if they are helpful (in this case I needed to know the difference between $() and ${}) regardless of whether they are _stellar_... — Ben Jones, Aug 07 '18 at 21:11
@BenJones Thanks for following up! I don't particularly mind the upvotes per se, I'm just baffled if this is a common question for people to have, or if this post on Stack Overflow is the one they are drawn to for an answer. (But I guess the title helps, and the distraction with a second unrelated question in the same post is easy to ignore, even if it's still a distraction.) — tripleee, Aug 08 '18 at 03:43

score 99 · Answer 2 · edited Dec 14 '14 at 19:42

99

$() 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 ${MY_VAR}/myFile.txt
```
will be interpreted as
```
echo toto/myFile.txt
```
Why can't I use it as bash$ 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.

edited Dec 14 '14 at 19:42

Jonathan Leffler

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answered Dec 14 '14 at 18:40

gturri

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"first evaluate this, and then evaluate the rest of the line" -- I don't understand this description of `$(...)`. – John Kugelman Dec 14 '14 at 19:43

score -1 · Answer 3 · answered Dec 14 '14 at 18:40

-1

your understanding is right. For detailed info on {} see bash ref - parameter expansion
'for' and 'while' have different syntax and offer different styles of programmer control for an iteration. Most non-asm languages offer a similar syntax.

With while, you would probably write i=0; while [ $i -lt 10 ]; do echo $i; i=$(( i + 1 )); done in essence manage everything about the iteration yourself

answered Dec 14 '14 at 18:40

Dinesh

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Not my down-vote, but the OP seems to be confusing `(command …)` with `$(commnd …)`, which are quite different in detail, though they share some commonality too. Thus, your first observation is not entirely accurate; the OP's understanding is not entirely correct. – Jonathan Leffler Dec 14 '14 at 19:48
3

_Most non-asm languages offer a similar syntax._ What??? – gniourf_gniourf Dec 14 '14 at 20:52
@gniourf_gniourf FOR syntax. In most non-assembly languages (in my limited knowledge), not counting some ancient ones like algol – Dinesh Dec 15 '14 at 00:44
@JonathanLeffler thanks for clarifying. I too thought so but figured that in the context OP had not meant a subshell. – Dinesh Dec 15 '14 at 00:48

Difference between ${} and $() in Bash

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