“a word that is identical to an alias being expanded is not expanded a second time”

Bash manual says

The first word of the replacement text is tested for aliases, but a
word that is identical to an alias being expanded is not expanded a
second time. This means that one may alias ls to ls -F, for
instance, and Bash does not try to recursively expand the replacement
text.

I’m trying to figure out which alias follows “identical to” in the quote,

• any alias being expanded in the same sequence of alias expansion recursions, or
• the alias whose expansion was first started, or
• the alias whose expansion was last started.

So I create an example

$ alias a1=a2; 
$ alias a2=a3;
$ alias a3=a4;

and want to check the alias expansion result of a1, in the following cases

$ alias a4=a1;

or

$ alias a4=a2;

or

$ alias a4=a3;

How can I check the alias expansion result of a1, possibly by performing alias expansion on a1 without letting the shell going further than alias expansion?

Answers:

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Method 1

What the manual says is that the shell will avoid any loop that may be caused by recursion of alias expansion.

With your example (a1=a2=a3=a4), if you execute alias a4=a1 you are creating a loop. Then, as soon as you will execute a1 (resp. a2, a3, a4), once the shell loops back to a1 (resp. a2, a3, a4) it will search for a command named a1 (resp. a2, a3, a4) that is NOT an alias (since that would create a never-ending loop).

Example:

$ a1() { echo Phew, I got out of the loop; }
$ alias a1='echo "(a1)"; a2' a2='echo "(a2)"; a3'
$ alias a3='echo "(a3)"; a4' a4='echo "(a4)"; a1'

$ a1
(a1)
(a2)
(a3)
(a4)
Phew, I got out of the loop

$ a2                              # Command a2 does not exist anywhere
(a2)
(a3)
(a4)
(a1)
a2: command not found

Method 2

:> alias e="echo "
:> alias text=foo
:> e text
foo

Technically the shell does go further than alias expansion but nonetheless I think this is what you meant.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0