Shell script learning for Linux systems

Linux system learning-shell script learning-Linux general technology-Linux programming and kernel information. The following is a detailed description. 1. (The use of colons)

: $ {Username = 'whoam'} # as a placeholder
:$ {HOSTNAME ?} $ {USER ?} $ {MAIL ?} # Set Evaluation Parameters
:> Data. xxx # file "data. xxx "is cleared now. This is the same as cat/dev/null> data. xxx works the same but does not generate new processes, because the colon is a built-in command.

2. ** is a power operation.

3. $? Exit status variable. $? Save the exit status of a command, function, or script.

$ Process ID variable. This $ variable saves the running script process ID

4. braces {xxx, yyy, zzz ...}

Braces extension, such:
Cat {file1, file2, file3}> combined_file
# Connect file1, file2, and file3 and redirect them to combined_file.
Cp file22. {txt, backup} # copy "file22.txt" to "file22.backup"

Echo {file1, file2 }\:{\ A, "B", 'C '}
# Output file1: A file1: B file1: C file2: A file2: B file2: C

Braces represent code blocks, but the variables can be seen outside. Variables in parentheses are irrelevant to the outside world.