Linux脚本中两个$有什么区别

Question

What is the difference between variable $processid ($) and Date=$(date +'%m-%d-%y')($) . Here we are using two kinds of $ symbols, please let me know what is the difference of those two $ .

Answer 1

From bash manual :

3.5.3 Shell Parameter Expansion
The '$' character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.
...
The basic form of parameter expansion is ${parameter}. The value of parameter is substituted.

Note, that braces { and } are optional...

...
3.5.4 Command Substitution
Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows:
$(command)
....
Bash performs the expansion by executing command in a subshell environment and replacing the command substitution with the standard output of the command, with any trailing newlines deleted.

So if:

a=1

then $a and ${a} expand to 1 .

The following:

a=$(echo 2)

executes the command echo with a single argument 2 in a subshell and grabs it's standard output ( echo 2 will write 2 with a newline on standard output). So it expands to:

a=2$'\n'

But trailing newlines are removed, so:

a=2

So ${there:1:2} ${are:-} ${many//.*/ways} ${to##use} ${the,,} ${dollar^^$} $(( ${!sign} + ${#in} )) ${!Bash*} .

Answer 2

Answer: 1.Date=$(date +'%m-%d-%y') -> this $ will execute the command(date +'%m-%d-%y')

2.$processid -> this $ will expand the Processid Variable.