On 6/18/25 11:38 AM, Alberto Millán wrote:
Bash Version: 5.2
Patch Level: 37
Release Status: release
Description:
When I define an alias and a function with the same name, I experience
abnormal behavior.
It seems to be especially dangerous when using a colon.
I think there is a misunderstanding about exactly when alias expansion
happens in the parsing process.
The current version of the man page says:
"Aliases allow a string to be substituted for a word that is in a posi-
tion in the input where it can be the first word of a simple command.
Aliases have names and corresponding values that are set and unset us-
ing the alias and unalias builtin commands (see SHELL BUILTIN COMMANDS
below).
If the shell reads an unquoted word in the right position, it checks
the word to see if it matches an alias name. If it matches, the shell
replaces the word with the alias value, and reads that value as if it
had been read instead of the word. The shell doesn't look at any char-
acters following the word before attempting alias substitution."
So as part of the tokenizing process, the shell reads words (and operators,
etc.), one at a time. If it reads a word, and it's in the position where
it could be the first word in a simple command, it tries alias expansion.
Given that, we can see what happens here.
alias x='echo foo'
x(){ :;}
# It return:
# -bash: syntax error near unexpected token `('
Because what you get in the input after alias expansion is basically
echo foo(){ :;}
which is a syntax error.
alias x='echo foo;'
x(){ :;}
# It return similar but diferent error:
# -bash: syntax error near unexpected token `)'
This time what you get is
echo foo;(){ :;}
Which is a valid simple command, followed by a left paren, which can
introduce a subshell or arithmetic command, followed by a right paren,
which is a syntax error.
alias x=:
x(){ :;} # Not error yet but...
x
# It return:
# Segmentation fault (core dumped)
You have managed to define and execute an infinitely recursive function:
: () { :; }
:
And when you put a colon at the end of the alias, the function
definition executes the alias.
```bash
alias x='id;:'
x(){ :;}
# It return the execution of the alias:
You have again managed to define a recursive function; this time, you
just executed a simple command before you did so.
bash -i -c $'alias x=:\n x(){ :;};x' # This crash.
bash -c $'alias x=:\n x(){ :;};x' # This do not crash.
bash -i -c $'alias x=:\nfunction x(){ :;};x' # This do not crash.
bash -i -c 'alias x=:; function x(){ :;};x' # This do not crash.
bash -i -c 'alias x=:; x(){ :;};x' # This do not crash.
Given the above, you should be able to figure out what these are doing.
Keep in mind that bash doesn't expand aliases by default in non-
interactive shells.
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU c...@case.edu http://tiswww.cwru.edu/~chet/
