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

Question

I cannot understand this line in one of the old scripts in my project:

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

Answer 1

This is a parameter extension pattern bash.

This statement indicates:

• If the value $1(the first positional parameter (argument)) is not set or is equal to zero, then the command output id -unwill be set as a variableUSER

• If the parameter is $1set and is not equal to zero, then the extension $1will be installed as a parameter USER.

, USER ​​ , , . script .

Parameter Expansion man bash, .

Answer 2

id (man id), id -un .

:- , , , . .

, , , , $USER $1, .

Answer 3

:

• ${} ,
• :- :-, ​​
• RH

:

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

, :

• ${parameter:?word} , ,

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

• ${parameter:+word} word, parameter

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

• ${parameter:=word} parameter word, parameter

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

Answer 4

USER:

USER=....

.

• " ", ${ }.
• $1. ${1 ... }.
• $1 , NUL , ${1:- }.
• $1 ( nul), .
• $1 NUL , -.
• - } " ".
• .
• id -un. , man id.
• id -un . , .

$(id -un).

:

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

USER , 1, , .

, :

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

If a value is specified, this value is returned:

$ tf bind
bind

If no username is specified, the name of the executable user is printed:

$ tf
bize