Substitution of shell variables, command substitution, escape character

Substitution of 1,shell variables

Variables can be replaced by a specific value depending on whether the variable is empty or deleted

${var} variable Original value

$ (Var:-word) If the variable is empty or has been deleted then return to Word, but do not change the value of Var

Name= "Xiaoming"

echo "Xiaoming ' s Age is ${age:-12}"

Ech "age = ${age}"

>>>>>>>>

Xiaoming ' s age is 12

Age =

>>>>>>>

$ (Var:=word) If the variable is empty or has been deleted then return to Word and change the value of Var

Name= "Xiaoming"

age=11

echo "Xiaoming ' s Age is ${age:+12}"

Ech "age = ${age}"

>>>>>>>>

Xiaoming ' s age is 12

Age = 12

>>>>>>

$ (Var:?word) If the variable is empty or has been deleted then return to Word, then send the message to the standard error output

Name= "Xiaoming"

echo "Xiaoming ' s Age is ${age:-12}"

Ech "age = ${age}"

>>>>>>>>

./hello.sh: Line 8:age:12

>>>>>>>

$ (Var:+word) If VAR is defined, then return to word, but do not change the value of Var

Example

Name= "Xiaoming"

age=11

echo "Xiaoming ' s Age is ${age:=12}"

Ech "age = ${age}"

>>>>>>>>

Xiaoming ' s age is 12

Age = 11

>>>>>>>

2, command substitution means that the shell can perform a command save and then output it in the appropriate place

Lll= ' ls ' note here is the anti-quote, not single quote
Echo ${lll}

3, escape character

\ \ counter Slash

\a Alarm Ringing

\b Backspace (delete key)

\f Page Change

\ nthe line break

\ r Enter

\ t Horizontal tab

\w Vertical Tab

Substitution of shell variables, command substitution, escape character