Linux Gvim Shell Replacement: variable substitution, command substitution, escape character

Replace: If the expression contains special characters, the shell will replace it. For example, using a variable in double quotes is a substitution. The escape character is also a replacement

#！ /bin/Basha=tenecho"Value of A is ${a} \ n"

The #-e indicates that the escape character is replaced with no output as follows #

echo "Value of A is ${a} \ n"

Escape character Substitution

The following escape characters are available for Echo

1. \ \ Backslash
2. \a Alarm, Bell?
3. \b Backspace (delete key)?
4. \f The page, move the current position to the beginning of the next page
5. \ n line break
6. \ r Enter
7. \ t One indent
8. \v Vertical tab?

Examples are as follows:

1 Echo " Value of A is ${a} \ \ " 2 Echo " Value of A is ${a} \ t ${a} "

You can suppress escaping by using the-e option of the Echo command, which is not escaped by default, or by using the-n option to prevent insertion of line breaks


Command substitution

The shell executes the command first, saves the output temporarily, and outputs it in the appropriate place.
The grammar ' command ' is the one under ESC.

Examples are as follows:

1Date= 'Date`2 Echo "Date is ${date}"3 4Users= 'W.H.O.|WC-l '5 Echo "logged in USR is $USERS"6 7Up= 'Date;Uptime`8 Echo "Uptime is ${up}"

Variable substitution

Can be based on the state of the variable (empty?) Defined? ) to change its value.
You can use the variable substitution form as follows
1. The original value of the ${var} variable
2. ${var:-word} If the variable var is empty or has been unset, return to Word without changing its value
3. ${var:=word} If the variable var is empty or has been unset, return to Word and set its value to Word
4. ${var:?message} If the variable var is empty or has been unset, then send the message to the standard error output, which can be used to detect whether Var can be assigned to a normal value
If this substitution appears in the shell script, the script will stop running
6. ${var:+word} If the variable var is defined, then return to word, but do not change its value

Examples are as follows:

1 Echo${var:-"Variable is not set"}2 Echo "1-value of Var is ${var}"3 4 unset var5 Echo${var:+"The is default value"}6 Echo "3-value of Var is ${var}"7 8 Echo${var:="Variable is not set"}9 Echo "2-value of Var is ${var}"Ten  OneVar="Chenmo" A Echo${var:?"Print This message"} #把ChenMo输出终端 # - Echo "4-value of Var is ${var}" -  theVar="Matto" - Echo${var:+"This is the default Valus"} - Echo "Value of Var is ${var}"

Linux Gvim Shell Replacement: variable substitution, command substitution, escape character