Bash – watch command alias expansion

aliasbash

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?

Best Answer

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.

Related Solutions

Bash Shell Quoting – Single or Double Quotes for Alias Definition?

In the example you mention, where the expansion for the alias is a single word containing no character that is subject to expansion, it makes no difference how you quote the name _cutf:

$ alias cutf="_cutf"
$ alias cutf
alias cutf='_cutf'

$ unalias cutf
$ alias cutf='_cutf'
$ alias cutf
alias cutf='_cutf'

$ unalias cutf
$ alias cutf=_cutf
$ alias cutf
alias cutf='_cutf'

As you can see in that interaction, recalling the value of the alias cutf gave the same result each time. So, yes, the quoting styles are interchangeable here.

It is not relevant that the expansion is to a function name: at least with bash, aliases perform a simple textual substitution (it is not obvious to me in what way defining an alias is useful; directly calling the underlying function seems just as easy).

Bash – command substitution within single quotes for alias

Single-quote vs double-quote versions

Let's define the alias using single-quotes:

$ alias d='$(date)'

Now, let's retrieve the definition of the alias:

$ alias d
alias d='$(date)'

Observe that no command substitution was yet performed.

Let's do the same, but this time with double-quotes:

$ alias d="$(date)"
$ alias d
alias d='Fri Oct 28 17:01:12 PDT 2016'

Because double-quotes are used, command substitution was performed before the alias was defined.

Single-quote version

Let's try executing the single-quote version:

$ alias d='$(date)'
$ d
bash: Fri: command not found

The single-quote version is equivalent to running:

$ $(date)
bash: Fri: command not found

In both cases, the command substitution is performed when the command is executed.

A variation

Let's consider this alias which uses command substitution and is defined using single-quotes:

$ alias e='echo $(date)'
$ e
Fri Oct 28 17:05:29 PDT 2016
$ e
Fri Oct 28 17:05:35 PDT 2016

Every time that we run this command, date is evaluated again. With single-quotes, the command substitution is performed when the alias is executed, not when it is defined.