Bash Alias – How to Force a Bash Word to Reference an Alias, Function, File, etc.

bash

When trying to add a bit more detail to a related question, I realized I don't know how to specify which one of the available Bash word types to execute. According to help type there are five word types:

alias
keyword
function
builtin
file

It's possible for a word to have several types (true is just an example; a more commonly overridden word would be cd):

$ type -a true
true is a shell builtin
true is /bin/true

How do you force the execution of a specific synonym? So far, in order of precedence:

alias: This is a special case since it has precedence over all the other synonyms. Forcing this would only be useful when the command should fail if the alias is not defined.
```
$ 
```
keyword:
```
$ 
```
function:
```
$ 
```
builtin:
```
$ builtin true
```

file:

$ command true
$ $(which true)
$ /bin/true

Partial solutions:

Quoting a word excludes aliases and keywords. That is,
```
$ 'true'
```
will run only the function, builtin or file.

Best Answer

What you're asking doesn't really make much sense.

keyword means it's a word that is part of the syntax of the shell. Those are recognised through tokenising. Quoting them is enough for the shell to stop recognising.

It is possible to alias a keyword in most shells though. So, aliases have precedence over keywords (in effect they are expanded early, and there is more tokenising after that where you can have more aliases, keywords)...

aliases (with the exception of zsh aliases defined with alias -g are only expanded in command position), so typically not in builtin the-alias.

functions have precedence over builtins, and builtins over external commands (and then $PATH decides which one to use).

You can force the builtin with:

builtin the-cmd and its args

(though it should be noted it is not a standard command).

You can disable aliases by quoting them (though the quoted version could also be aliased in some shells).

So, for instance (here zsh syntax), in:

'while'() echo function while
alias 'while=echo alias while'

Writing while true would actually output alias while true, and there would be no way to use the while keyword since quoting it would disable both the alias and keyword.

You would call the while function with for instance:

'whi'le whatever

If there was a while builtin (but of course their wouldn't in those shells that have while keyword), you'd write it:

builtin while whatever

And to call the while command, you'd write:

env while whatever

or in zsh (when not in sh emulation):

command while whatever

(in other shells, command only prevents functions, not builtins)

/full/path/to/while whatever

Of course, nothing's stopping you from doing even sillier things like:

alias 'while=while "w"hile; do'
"while"() { while "whi"le; done; }

Which, at least in zsh is valid (but stupid).

Why the alias doesn't work

alias wd='ps -ef | grep java | awk {'print $2 " " $9'} | egrep "(A|B|C|D)"'

The alias command receives three arguments. The first is the string wd=ps -ef | grep java | awk {print (the single quotes prevent the characters between them from having a special meaning). The second argument consists of a single space character. (In .bashrc, the positional parameters $2 and $9 are empty, so $2 expands to a list of 0 words.) The third argument is } | egrep "(A|B|C|D)" (again the single quotes protect the special characters).

The alias definition is parsed like any other shell command when it is encountered. Then the string defined for the alias is parsed when the alias is expanded. Here are some possible ways to define this alias. First possibility: since the whole alias definition is within single quotes, only use double quotes in the commands, which means you must protect the " and $ meant for awk with backslashes.

alias wd='ps -ef | grep java | awk "{print \$2 \" \" \$9}" | egrep "(A|B|C|D)"'

Second possibility: every character stands for itself within single quotes, except that a single quote ends the literal string. '\'' is an idiom for “single quote inside a single-quoted string”: end the single-quoted string, put a literal single quote, and immediately start a new single-quoted string. Since there's no intervening space, it's still the same word.

alias wd='ps -ef | grep java | awk '\''{print $2 " " $9}'\'' | egrep "(A|B|C|D)"'

You can simplify this a bit:

alias wd='ps -ef | grep java | awk '\''{print $2, $9}'\'' | egrep "(A|B|C|D)"'

Tip: use set -x to see how the shell is expanding your commands.

Why the function doesn't work

I don't know. The part you show looks ok. If you still don't understand why your function isn't working after my explanations, copy-paste your code.

Alias or function?

Use an alias only for very simple things, typically to give a shorter name to a frequently-used command or provide default options. Examples:

alias grep='grep --color'
alias cp='cp -i'
alias j=jobs

For anything more complicated, use functions.

What you should have written

Instead of parsing the ps output, make it generate output that suits you.

wd () {
  ps -C java -o pid=,cmd= | egrep "(A|B|C|D)"
}

Bash – Writing bash function as one-line alias

As @AndreasWiese comments, you can't do this with an alias. Aliases tack on arguments space-separated, so you can't join them (in a simple way) to the aliased command.

A comment on your function: Why are you using $@? You should use $1 instead. If you call your function with multiple arguments (p a b c), you'll probably get ~~an error (b: command not found)~~ an unknown directory.

Best Answer

Related Solutions

Bash – Troubleshooting Alias or Function Issues

Why the alias doesn't work

Why the function doesn't work

Alias or function?

What you should have written

Bash – Writing bash function as one-line alias

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