tl;dr
A sole stuff
would most probably work for you.
Full answer
What happens
When you run foo $(stuff)
, this is what happens:
stuff
runs;
- its output (stdout), instead of being printed, replaces
$(stuff)
in the invocation of foo
;
- then
foo
runs, its command line arguments obviously depend on what stuff
returned.
This $(…)
mechanism is called "command substitution". In your case the main command is echo
which basically prints its command line arguments to stdout. So whatever stuff
tries to print to stdout is captured, passed to echo
and printed to stdout by echo
.
If you want the output of stuff
to be printed to stdout, just run the sole stuff
.
The `…`
syntax serves the same purpose as $(…)
(under the same name: "command substitution"), there are few differences though, so you cannot blindly interchange them. See this FAQ and this question.
Should I avoid echo $(stuff)
no matter what?
There is a reason you may want to use echo $(stuff)
if you know what you're doing. For the same reason you should avoid echo $(stuff)
if you don't really know what you're doing.
The point is stuff
and echo $(stuff)
are not exactly equivalent. The latter means calling split+glob operator on the output of stuff
with the default value of $IFS
. Double quoting the command substitution prevents this. Single quoting the command substitution makes it no longer be a command substitution.
To observe this when it comes to splitting run these commands:
echo "a b"
echo $(echo "a b")
echo "$(echo "a b")" # the shell is smart enough to identify the inner and outer quotes
echo '$(echo "a b")'
And for globbing:
echo "/*"
echo $(echo "/*")
echo "$(echo "/*")" # the shell is smart enough to identify the inner and outer quotes
echo '$(echo "/*")'
As you can see echo "$(stuff)"
is equivalent(-ish*) to stuff
. You could use it but what's the point of complicating things this way?
On the other hand if you want the output of stuff
to undergo splitting+globbing then you may find echo $(stuff)
useful. It has to be your conscious decision though.
There are commands generating output that should be evaluated (which includes splitting, globbing and more) and run by the shell, so eval "$(stuff)"
is a possibility (see this answer). I have never seen a command that needs its output to undergo additional splitting+globbing before being printed. Deliberately using echo $(stuff)
seems very uncommon.
What about var=$(stuff); echo "$var"
?
Good point. This snippet:
var=$(stuff)
echo "$var"
should be equivalent to echo "$(stuff)"
equivalent(-ish*) to stuff
. If it's the whole code, just run stuff
instead.
If, however, you need to use the output of stuff
more than once then this approach
var=$(stuff)
foo "$var"
bar "$var"
is usually better than
foo "$(stuff)"
bar "$(stuff)"
Even if foo
is echo
and you get echo "$var"
in your code, it may be better to keep it this way. Things to consider:
- With
var=$(stuff)
stuff
runs once; even if the command is fast, avoiding computing the same output twice is the right thing. Or maybe stuff
has effects other than writing to stdout (e.g. creating a temporary file, starting a service, starting a virtual machine, notifying a remote server), so you don't want to run it multiple times.
- If
stuff
generates time-depending or somewhat random output, you may get inconsistent results from foo "$(stuff)"
and bar "$(stuff)"
. After var=$(stuff)
the value of $var
is fixed and you can be sure foo "$var"
and bar "$var"
get identical command line argument.
In some cases instead of foo "$var"
you may want (need) to use foo $var
, especially if stuff
generates multiple arguments for foo
(an array variable may be better if your shell supports it). Again, know what you're doing. When it comes to echo
the difference between echo $var
and echo "$var"
is the same as between echo $(stuff)
and echo "$(stuff)"
.
*Equivalent(-ish)?
I said echo "$(stuff)"
is equivalent(-ish) to stuff
. There are at least two issues that make it not exactly equivalent:
$(stuff)
runs stuff
in a subshell, so it's better to say echo "$(stuff)"
is equivalent(-ish) to (stuff)
. Commands that affect the shell they run in, if in a subshell, don't affect the main shell.
In this example stuff
is a=1; echo "$a"
:
a=0
echo "$(a=1; echo "$a")" # echo "$(stuff)"
echo "$a"
Compare it with
a=0
a=1; echo "$a" # stuff
echo "$a"
and with
a=0
(a=1; echo "$a") # (stuff)
echo "$a"
Another example, start with stuff
being cd /; pwd
:
cd /bin
echo "$(cd /; pwd)" # echo "$(stuff)"
pwd
and test stuff
and (stuff)
versions.
echo
is not a good tool to display uncontrolled data. This echo "$var"
we were talking about should have been printf '%s\n' "$var"
. But since the question mentions echo
and since the most probable solution is not to use echo
in the first place, I decided not to introduce printf
up until now.
stuff
or (stuff)
will interleave stdout and stderr output, while echo $(stuff)
will print all the stderr output from stuff
(which runs first), and only then the stdout output digested by echo
(which runs last).
$(…)
strips off any trailing newline and then echo
adds it back. So echo "$(printf %s 'a')" | xxd
gives different output than printf %s 'a' | xxd
.
Some commands (ls
for example) work differently depending if the standard output is a console or not; so ls | cat
does not the same ls
does. Similarly echo $(ls)
will work differently than ls
.
Putting ls
aside, in a general case if you have to force this other behavior then stuff | cat
is better than echo $(ls)
or echo "$(ls)"
because it doesn't trigger all the other issues mentioned here.
Possibly different exit status (mentioned for completeness of this wiki answer; for details see another answer that deserves credit).
11
It's redundant and therefore should not be used. Compare with Useless Use Of Cat: http://porkmail.org/era/unix/award.html
– dr01 – 2018-08-28T08:20:11.4277
echo $(echo $(echo $(echo$(echo $(stuff)))))
also does work, and still you probably wouldn't use it, right? ;-) – Peter - Reinstate Monica – 2018-08-29T07:57:26.5531What are you trying to do with that expansion, exactly? – curiousguy – 2018-08-29T08:08:25.083
@curiousguy I'm trying to help other users, hence my answer below. Recently, I've seen at least three questions that use this syntax. Their authors were trying to do… various
stuff
. :D – Kamil Maciorowski – 2018-08-29T08:15:11.6972Also, don't we all agree that it's unwise to confine oneself to an echo chamber?? ;-) – Peter - Reinstate Monica – 2018-08-29T08:24:06.810
Does the tag [tag:sh] imply a strictly
/bin/sh
question? – curiousguy – 2018-08-29T08:26:10.797@curiousguy "The standard command language interpreter". I added this to indicate shells compatible and/or derived from
sh
(e.g. not [tag:Powershell]). It's not about any single shell; it's about the concept of usingecho
with command substitution this way. – Kamil Maciorowski – 2018-08-29T08:38:49.517@KamilMaciorowski There are mostly compatible shells with support for all basic constructs but with sometimes different semantics.
zsh
is interesting. – curiousguy – 2018-08-29T09:12:25.753