Change order of expansions(bash)

Question

I need parameter expansion after command substitution.

GNU bash, version 4.1.5(1)

$ foo() { echo \$a; }
$ a=5
$ echo $(foo)
$a

Is it possible?

test:

#!/bin/bash

echo $a

run:

a=5
echo $(./test)

if test:

#!/bin/bash

echo \$a

run:

echo $(./test)
$a

Don't work(

Didn't get your question? Can you clarify? You want that the result of echo is the contents of a? Remove the backslash! — rems, Feb 25 '11 at 12:34

score 2 · Answer 1 · answered Feb 25 '11 at 12:30

Order can't be changed without changing source code.

You can use eval:

eval echo $(foo)

man bash:

   The order of expansions is: brace expansion, tilde  expansion,  parame‐
   ter,  variable  and arithmetic expansion and command substitution (done
   in a left-to-right fashion), word splitting, and pathname expansion.

score 2 · Answer 2 · answered Feb 25 '11 at 12:30

2

... What?

$ foo() { echo $a; }
$ a=42
$ echo $(foo)
42

answered Feb 25 '11 at 12:30

Ignacio Vazquez-Abrams

45,019
5
78
84

@Ignacio Don't work, I update question – osdyng Feb 25 '11 at 12:37
Of course your first test doesn't work. You forgot to export the variable. – Ignacio Vazquez-Abrams Feb 25 '11 at 12:41
@Ignacio how can I do? – osdyng Feb 25 '11 at 12:45
With `export a=5`. – Ignacio Vazquez-Abrams Feb 25 '11 at 12:45
@Ignacio It work only if inner command expand variable. python don't expand output. I also need use eval. – osdyng Feb 25 '11 at 14:13
Then your question is fallacious, since that has nothing to do with order of expansion. – Ignacio Vazquez-Abrams Feb 25 '11 at 14:33

score 1 · Answer 3 · answered Feb 25 '11 at 12:36

1

echo `eval foo`

Is this what you want?

answered Feb 25 '11 at 12:36

rems

2,240
13
11

echo `eval $(./test)` 5: command not found – osdyng Feb 25 '11 at 12:46
Should be "echo $(eval foo)" or as I wrote "echo \`eval foo\`". – rems Feb 25 '11 at 14:57

score 0 · Answer 4 · answered Feb 25 '11 at 17:42

Function:

$ a=5
$ foo() { echo \$a; }
$ foo
$a
$ eval echo $(foo)
5

Script:

#!/bin/bash
echo $a

Run:

$ a=5
$ ./test
$ export a
$ ./test
5

New script:

#!/bin/bash
echo 'echo $a'

Run:

$ a=5
$ ./test
echo $a
$ eval $(./test)
5

Python example:

$ a=5
$ python -c 'print "echo $a"'
echo $a
$ eval $(python -c 'print "echo $a"')

Another Python example:

$ a=5
$ python -c 'print "a=42"'
a=42
$ echo $a
5
$ declare $(python -c 'print "a=42"')
$ echo $a
42

Change order of expansions(bash)

4 Answers4