How to get the output of a shell function without forking a sub shell?

Question

I have the following functions.

hello () {
        echo "Hello"
}
func () {
        hello
        echo "world"
}

If I don't want the output of the hello function to be printed but want to do something with it, I want to capture the output in some variable, Is the only possible way is to fork a subshell like below? Is it not an unnecessary creation of a new child process? Can this be optimized?

func () {
        local Var=$(hello)
        echo "${Var/e/E} world"
}

It may be a creation of a subshell, but why is it a problem? Are you sure you're not optimizing prematurely? — evil otto, Sep 22 '11 at 04:26
@evilotto It may be a problem if function has side effects besides outputting to stdout: for example, changing variables. — ZyX, Sep 22 '11 at 16:01

score 4 · Accepted Answer · answered Sep 21 '11 at 16:39

4

An ugly solution is to temporarily replace echo so that it sets a global variable, which you can access from your function:

func () {
  echo () {
    result="$@"
  }
  result=
  hello
  unset -f echo
  echo "Result is $result"
}

I agree it's nasty, but avoids the subshell.

answered Sep 21 '11 at 16:39

Idelic

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I was also thinking on similar lines. I can replace all echo with Echo which does something like this. Will this make the shell functions faster? – balki Sep 21 '11 at 16:46

codeforester · Answer 2 · 2018-05-07T18:24:24.013

You can make the caller pass in a variable name to hold the output value and then create a global variable with that name inside the function, like this:

myfunc() { declare -g $1="hello"; }

Then call it as:

myfunc mystring
echo "$mystring world" # gives "hello world"

So, your functions can be re-written as:

hello() {
    declare -g $1="Hello"
}

func() {
    hello Var
    echo "${Var/e/E} world"
}

The only limitation is that variables used for holding the output values can't be local.

Related post which talks about using namerefs:

How to return an array in bash without using globals?

score 2 · Answer 3 · answered Mar 04 '18 at 23:26

2

Not a bash answer: At least one shell, ksh optimises command substitution $( ... ) to not spawn a subshell for builtin commands. This can be useful when your script tends to perform a lot of these.

answered Mar 04 '18 at 23:26

Henk Langeveld

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bash appears to do the same – Laurence Renshaw Aug 06 '18 at 03:25
1

@LaurenceRenshaw In bash 4.4.12(1) on linux, `echo $(echo foo)` calls clone(), according to strace. That makes me think it *does* spawn a subshell. Do you have evidence that it doesn't? – Don Hatch Feb 12 '19 at 02:49
It is a child process, but not a full sub-shell. The child does not call `exec()`, so does not incur the full overhead of a sub-shell, but it is still a separate process. – Andrew Vickers Oct 18 '19 at 20:30
@AndrewVickers The ksh93 shell does not always fork for `$( ... )`, so no new process is created. – Henk Langeveld Oct 18 '19 at 21:03
@Hank. Good to know for ksh, but apparently not the case for bash. – Andrew Vickers Oct 18 '19 at 21:09

score 2 · Answer 4 · answered Sep 22 '11 at 18:22

2

How about using a file descriptor and a Bash here string?

hello () {
    exec 3<<<"Hello"
}

func () {
    local Var
    exec 3>&-
    hello && read Var <&3
    echo "${Var/e/E} world"
    exec 3>&-
}

func

answered Sep 22 '11 at 18:22

tim

21
1

Bash cryptics are practically endless,so4the~99% of us who even as long-time Bash coders NOT know-else-memomorize 100% of Bash docs, IN ENGLISH,"recode called fn so all its output is to a file-descriptor OTHER than stdout then manipulate that"(advanced!),per relevant OFFICIAL URLs FOR *ADVANCED* Bash ops http://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html#index-exec & http://www.gnu.org/software/bash/manual/html_node/Redirections.html#Duplicating-File-Descriptors tho no find defined there quote(>&-) tho I do at http://www.tldp.org/LDP/abs/html/io-redirection.html#CFD – Destiny Architect Jun 11 '14 at 06:05
--Advanced! But is there a way (put in another answer here) to do this as the asker apparently asked(I really want,too): WITHOUT having to recode the function being called(so say in this case function remains hello(){ echo "Hello" })? -but still NOT creating a sub-shell(so then allowing the fn to do visible environment side-effects&more benefits),as called fn's stdout still redirected to this other file-descriptor. I made attempts such as my (internal)ID N6ZPBM where quote(exec hello 1>&3) (see my last cmt for official defs of these ops),but sadly these just give quote(3: Bad file descriptor). – Destiny Architect Jun 11 '14 at 06:24

Andrew Vickers · Answer 5 · 2019-10-18T20:35:51.813

1

Do you have the option of modifying the hello() function? If so, then give it an option to store the result in a variable:

#!/bin/bash

hello() {
  local text="hello"

  if [ ${#1} -ne 0 ]; then
    eval "${1}='${text}'"
  else
    echo "${text}"
  fi
}

func () {
  local var     # Scope extends to called functions.
  hello var
  echo "${var} world"
}

And a more compact version of hello():

hello() {
  local text="hello"
  [ ${#1} -ne 0 ]  && eval "${1}='${text}'" || echo "${text}"
}

edited Oct 18 '19 at 20:35

answered Oct 18 '19 at 19:55

Andrew Vickers

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Actually, why even bother with `-v`. If there is a parameter, we can assume it is the variable name. Edited accordingly. – Andrew Vickers Oct 18 '19 at 20:33
Fantastic! This works in a primitive BusyBox interpreter too. – BuvinJ Oct 21 '19 at 13:20

score 0 · Answer 6 · answered Oct 21 '19 at 15:01

This doesn't literally answer the question, but it is a viable alternate approach for some use cases...

This is sort of a spin off from @Andrew Vickers, in that you can lean on eval.

Rather than define a function, define what I'll call a "macro" (the C equivalent):

MACRO="local \$var=\"\$val world\""

func()
{ 
    local var="result"; local val="hello"; eval $MACRO; 
    echo $result; 
}

How to get the output of a shell function without forking a sub shell?

6 Answers6