Printf and shell touch

Formatting: printf'print format' actual content
What are the characters represented by the value 45?
Printf'/x45/N' Note:/xnn is a two-digit number, which can be converted into characters.

Introduction to SED and awk tools:
You can analyze the standard input data and output it to the standard output after processing. It supports replacement, deletion, addition, and selection of specific rows.
Sed is often used for processing the entire row, while awk tends to process the number "field" in one row, because awk is suitable for processing small data.
Sed [-nefr] [action]
Awk 'condition type 1 {Action 1} Condition Type 2 {Action 2} ...... 'filename

Test can test whether a file exists and is of great use in shell programming.
[Samson @ yygy] $ test-e mutex. c
[Samson @ yygy] $ test-e mutex. C & Echo "yes" | echo "no"
Yes