How to enable `sudo` with custom functions

aliasfunctionsudo

Recently I learned you can enable sudo for custom aliases as follows:

alias sudo='sudo ' # note: the space is required!

The reason this works is the following:

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

My question is: is there any way to enable sudo with custom functions too?

Best Answer

In the general case what you're trying to do doesn't really work.

With aliases, there is pretty much just a string rewrite before the line is passed for execution.

So, for example, if you have

alias sudo='sudo '
alias foo='bar baz'

then when you enter sudo foo the command line is rewritten to sudo bar baz and that is then what is run. This is simple command line rewriting.

Now functions are harder. They're not simple rewrites, but a complete evaluation; they can set variables, change directories, open files... pretty much do anything the shell can do. And, importantly, they're run in the context of the current shell. When you run sudo myfunction then none of this is possible; in particular sudo commands are run as a sub-process and so can't affect the current shell.

The work-around used for things like the sudowrap mentioned above is to try and automatically work out sudo bash -c 'myfunction() {...} ; myfunction'. This explicitly calls a new bash subshell and then runs the function in that subshell. The explicit call makes it clear that things like setting variables and the like won't work :-) It allows for a limited subset of functionality.

The sort of functions you can call this way may be better rewritten as shell scripts rather than functions; then sudo can call them directly. The example given at http://w00tbl0g.blogspot.com/2007/05/using-bash-functions-under-sudo.html would be easier by having duk converted to an executable

#!/bin/bash
exec du -k "$@" | sort -n

That'll then work as expected!

Related Solutions

When to Use Alias, Script, or Function in Bash

An alias should effectively not (in general) do more than change the default options of a command. It is nothing more than simple text replacement on the command name. It can't do anything with arguments but pass them to the command it actually runs. So if you simply need to add an argument at the front of a single command, an alias will work. Common examples are

# Make ls output in color by default.
alias ls="ls --color=auto"
# make mv ask before overwriting a file by default
alias mv="mv -i"

A function should be used when you need to do something more complex than an alias but that wouldn't be of use on its own. For example, take this answer on a question I asked about changing grep's default behavior depending on whether it's in a pipeline:

grep() { 
    if [[ -t 1 ]]; then 
        command grep -n "$@"
    else 
        command grep "$@"
    fi
}

It's a perfect example of a function because it is too complex for an alias (requiring different defaults based on a condition), but it's not something you'll need in a non-interactive script.

If you get too many functions or functions too big, put them into separate files in a hidden directory, and source them in your ~/.bashrc:

if [ -d ~/.bash_functions ]; then
    for file in ~/.bash_functions/*; do
        . "$file"
    done
fi

A script should stand on its own. It should have value as something that can be re-used, or used for more than one purpose.

Bash – How to document the custom bash functions and aliases

To list active aliases, run:

alias

To see names of all active functions, run:

declare -F

To see the names and definitions of all active functions, run:

declare -f

The information on aliases is also available is a script-friendly format with:

declare -p BASH_ALIASES

man bash provides more info on the alias builtin:

   alias [-p] [name[=value] ...]
          Alias with  no  arguments  or  with  the  -p
          option  prints  the  list  of aliases in the
          form alias name=value  on  standard  output.
          When  arguments  are  supplied,  an alias is
          defined for each name whose value is  given.
          A  trailing  space in  value causes the next
          word to be checked  for  alias  substitution
          when  the  alias is expanded.  For each name
          in the argument list for which no  value  is
          supplied, the name and value of the alias is
          printed.  Alias returns true unless  a  name
          is   given  for  which  no  alias  has  been
          defined.

Regarding functions, man bash explains that declare can provide still more information is available if the extdebug option is set:

   Function  names  and definitions may be listed with
   the -f option to the  declare  or  typeset  builtin
   commands.  The -F option to declare or typeset will
   list the function names only  (and  optionally  the
   source  file and line number, if the extdebug shell
   option is enabled).

Best Answer

Related Solutions

When to Use Alias, Script, or Function in Bash

Bash – How to document the custom bash functions and aliases

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