Alias and functions

From the bash manual

The rules concerning the definition and use of aliases are somewhat
confusing. Bash always reads at least one complete line of input
before executing any of the commands on that line. Aliases are
expanded when a command is read, not when it is executed. Therefore,
an alias definition appearing on the same line as another command does
not take effect until the next line of input is read. The commands
following the alias definition on that line are not affected by the
new alias. This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a
compound command. As a consequence,
aliases defined in a function are not available until after that function is executed. To be safe, always put alias definitions on a
separate line, and do not use alias in compound commands.

The two sentences “Aliases are expanded when a function definition is read, not when the function is executed” and “aliases defined in a function are not available until after that function is executed” seem to be contrary to each other.

Can you explain what they mean respectively?

Answers:

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Method 1

1. Aliases are expanded when a function definition is read,
   not when the function is executed …

   $ echo "The quick brown fox jumps over the lazy dog." > myfile
    
   $ alias myalias=cat
    
   $ myfunc() {
   >     myalias myfile
   > }
    
   $ myfunc
   The quick brown fox jumps over the lazy dog.
    
   $ alias myalias="ls -l"
    
   $ myalias myfile
   -rw-r--r-- 1 myusername mygroup 45 Dec 13 07:07 myfile
    
   $ myfunc
   The quick brown fox jumps over the lazy dog.

   Even though myfunc was defined to call myalias,
   and I’ve redefined myalias,
   myfunc still executes the original definition of myalias. 
   Because the alias was expanded when the function was defined. 
   In fact, the shell no longer remembers that myfunc calls myalias;
   it knows only that myfunc calls cat:

   $ type myfunc
   myfunc is a function
   myfunc ()
   {
   cat myfile
   }

2. … aliases defined in a function are not available
   until after that function is executed.

   $ echo "The quick brown fox jumps over the lazy dog." > myfile
    
   $ myfunc() {
   >     alias myalias=cat
   > }
    
   $ myalias myfile
   -bash: myalias: command not found
    
   $ myfunc
    
   $ myalias myfile
   The quick brown fox jumps over the lazy dog.

   The myalias alias isn’t available
   until the myfunc function has been executed. 
   (I believe it would be rather odd
   if defining the function that defines the alias
   was enough to cause the alias to be defined.)

Method 2

I need the answer that is stated by the first sentence when I try below snippet in my .bashrc.

alias ls='ls -F --color=auto --show-control-chars'
alias ll='ls -ahl'
function lf_macro() {
    local CMD=${1:-ls} DIR=${2:-.};
    $CMD $(find $DIR -maxdepth 1 -type f);
}
function lf() { lf_macro ll "$1"; }
function lsf() { lf_macro ls "$1"; }     # list all file, no directories

after unalias -a; source ~/.bashrc, I try to execute lf and lsf,

$ lf
-bash: ll: command not found

$ lsf
./file1 ./file2 ./script.sh ...     # no color, no control-chars

$ ls $(find -maxdepth 1 -type f)
./file1 ./file2 ./script.sh* ...

it seems clearly that aliases are expanded at function definition, not function execution, since:

• when I execute lf, the error -bash: ll: command not found, and
• when I execute lsf, /usr/bin/ls is used, not the alias form, no color
  highlight, and no control chars after executable file.

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