Ignored alias inside for loop

Ignored alias inside for loop

[Rusty Bird]

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Hash: SHA512

Hi,

Why does this ...

#!/bin/sh
for i in 1; do
    alias foobarbaz='echo ok'
    foobarbaz
done

... result in "foobarbaz: not found", but without the for loop it works?
Maybe I'm missing something in the spec, because bash-as-sh behaves
the same.

Rusty
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Re: Ignored alias inside for loop

[Jilles Tjoelker]

On Sun, Jul 26, 2015 at 09:23:52AM +0000, Rusty Bird wrote:
> Why does this ...

> #!/bin/sh
> for i in 1; do
>     alias foobarbaz='echo ok'
>     foobarbaz
> done

> ... result in "foobarbaz: not found", but without the for loop it works?
> Maybe I'm missing something in the spec, because bash-as-sh behaves
> the same.

Aliases are expanded during parsing, not during execution (like
functions are). The for loop is parsed completely before it is executed.

Using a function instead of an alias, or using eval will do what you
want.

Note that some parts of the parse/execute split are not specified by
POSIX. In particular, ksh93 and the real Bourne shell parse dot scripts
completely before executing anything, while most other shells parse as
needed. Example: given the file alias1.sh below:

alias foobarbaz='echo ok'
foobarbaz

The result is ok when running  dash -c '. ./alias1.sh'  but a command
not found error message when running  ksh93 -c '. ./alias1.sh'.

Similarly, a string as passed to the -c option or eval or trap builtins
may or may not be parsed fully before execution.

-- 
Jilles Tjoelker

Re: Ignored alias inside for loop

[Rusty Bird]

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Hash: SHA512

Hi Jilles,

> Aliases are expanded during parsing, not during execution (like 
> functions are). The for loop is parsed completely before it is
> executed.

Ah, what confused me was that I wrongly remembered it working inside
an if (instead of for) block. But looking again now, only the alias
*definition* was inside.

> Using a function instead of an alias, or using eval will do what
> you want.

Do you mean replacing the line with: eval "alias foobarbaz='echo ok'"
That doesn't seem to work either (in dash or bash).

> Note that some parts of the parse/execute split are not specified
> by POSIX. In particular, ksh93 and the real Bourne shell parse dot
> scripts completely before executing anything, while most other
> shells parse as needed. Example: given the file alias1.sh below:
> 
> alias foobarbaz='echo ok' foobarbaz
> 
> The result is ok when running  dash -c '. ./alias1.sh'  but a
> command not found error message when running  ksh93 -c '.
> ./alias1.sh'.
> 
> Similarly, a string as passed to the -c option or eval or trap
> builtins may or may not be parsed fully before execution.

Interesting, thanks!

Rusty
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Re: Ignored alias inside for loop

[Jilles Tjoelker]

On Sun, Jul 26, 2015 at 04:09:12PM +0000, Rusty Bird wrote:
> > Aliases are expanded during parsing, not during execution (like 
> > functions are). The for loop is parsed completely before it is
> > executed.

> Ah, what confused me was that I wrongly remembered it working inside
> an if (instead of for) block. But looking again now, only the alias
> *definition* was inside.

> > Using a function instead of an alias, or using eval will do what
> > you want.

> Do you mean replacing the line with: eval "alias foobarbaz='echo ok'"
> That doesn't seem to work either (in dash or bash).

eval foobarbaz

This may not be practical since it needs to be done for every alias use.

-- 
Jilles Tjoelker

Re: Ignored alias inside for loop

[Seb]

On Sun, Jul 26, 2015 at 04:09:12PM +0000, Rusty Bird wrote:

Hello,

> > Using a function instead of an alias, or using eval will do what
> > you want.
> 
> Do you mean replacing the line with: eval "alias foobarbaz='echo ok'"
> That doesn't seem to work either (in dash or bash).

In this case, where there is no further expansion to be done in the
alias content, you could use a mere variable:

  foobarbaz='echo ok'
  $foobarbaz "$@"

Or, if some further expansion has to be made, 'eval' again:

  foobarbaz='echo $PATH'
  eval $foobarbaz '"$@"'

++
Seb.