watch command alias expansion

Question

If a run the watch command containing an alias, it will not expand the alias. I have tried both with single quote and double quotes, in fact given the following alias:

# alias ll
alias ll='ls -l --color=tty'

The following command will fail

# watch ll
sh: ll: command not found

Shouldn't command line expansion work in this case?

Answer 1

Aliases are only expanded as the first argument, or after another alias with a trailing space on the end of the command.

From bash's help alias:

A trailing space in VALUE causes the next word to be checked for alias substitution when the alias is expanded.

To do this, try the following:

alias watch='watch '
alias ll='ls -l --color=tty'
watch ll

Bear in mind that some versions of watch strip colours by default, on some versions this can be stopped by using --color or -G.

For some other situations where this is inconvienient, you might want to use an array instead, for example:

ll=(ls -l --color=tty)

# Each of the program arguments are passed separately to watch
watch "${ll[@]}"
# All of the program arguments are passed as one argument to watch
watch "${ll[*]}"

Answer 2

I thought here basic problem is while executing watch, the argument is given to "sh -c" which means if alias ll is not defined in sh(dash shell's) environment then it will not expand it. But I was wrong and this is not the case, Chris' answer above is right.

Answer 3

man bash:

ALIASES
...
If the last character of the alias value is a blank, then the
next command word following the alias is also checked for alias
expansion.

...
For almost every purpose, aliases are superseded by shell functions.