CODEWITHSUNDEEP
bash
Sean Lin
Sean Lin

Reputation: 41

What does this bash mean: USER=${1:-`id -un`}

I can not understand this line in one of old scripts in my project :

USER=${1:-`id -un`}

Upvotes: 4

Views: 3252

Answers (4)

user2350426
user2350426

Reputation:

That line is setting the value of a variable called USER:

USER=....

The right side of the equal has several levels.

  • A "parameter expansion", the ${ }.
  • The parameter being expanded is $1. That's why it is ${1 ... }.
  • The value of $1 is being tested if it is NUL or unset, the ${1:- }.
  • If there is a value of $1 (other than nul), that value is substituted.
  • If $1 is NUL or unset, substitute the value at the rigth of the -.
  • The value between the - and the } is a "command substitution".
  • The "command execution" executes a command and replace its output.
  • The command is id -un. You can understand what it does with man id.
  • The command id -un prints a name. The current user if user is omitted.

The same command could be executed by $(id -un).

That would change the line to this:

$ USER=${1:-$(id -un)}

Which will set USER to the name given as positional parameter 1 or the name of the current user executing the line.

To try it, you can make a function:

$ tf(){ USER=${1:-$(id -un)}; echo "$USER"; }

If an value is given, that value is printed back:

$ tf bind
bind

If a user-name is not given, the executing user name is printed:

$ tf
bize

Upvotes: 1

drewyupdrew
drewyupdrew

Reputation: 1609

Looking at the manual page for id (man id), the command id -un will return the actual name of the user.

The format :- uses what's on the right only if what's on the left isn't set. More can be learned about this syntax here.

Therefore, the code you provided is essentially saying default to user, but override the $USER variable to the value of $1 if it is provided.

Upvotes: 2

dawg
dawg

Reputation: 104015

Parse it this way:

  1. The ${} tells you it is a parameter expansion
  2. The :- is use a default value of the Right Hand of the :- if the Left Hand is unset
  3. The backticks around the RH runs that command

Example:

$ sv="set v"
$ echo ${sv:-`whoami`}
set v
$ echo ${not_set:-`whoami`}
dawg

There are other forms, including:

  1. ${parameter:?word} display an error if the parameter is unset,

    $ echo ${not_set:?'is not set to anything'}
    -bash: not_set: is not set to anything

  2. ${parameter:+word} substitute value of word only if parameter is set

    $ echo ${sv:+'substitute for sv'}
    substitute for sv

  3. ${parameter:=word} assign parameter the value of word if parameter is unset

    $ echo ${not_set:='now it IS set'}
    now it IS set
    $ echo "$not_set"
    now it IS set

Upvotes: 1

heemayl
heemayl

Reputation: 42087

It's a bash parameter expansion pattern.

The given statement indicates:

  • If the value of $1 (the first positional parameter (argument)) is unset or null then the output of the command id -un will be set as variable USER

  • If the parameter $1 is set and not null then the expansion of $1 will be set as parameter USER.

Also the variable USER should be set session wide upon login, unless you have a very good reason you should not modify it directly. You can use a different variable name in your script as a solution then.

Check the Parameter Expansion section of man bash to get more idea.

Upvotes: 5

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