if the actual production work statement is the most important and most commonly used statement, therefore, we must firmly grasp
if conditions grammar
1. Single branch
if [condition]
then
instruction
be
or
if [Condition]; then
instruction
be
And previously studied the expression can be converted to each other
Two-branch structure
grammar:
if [condition]
then
Instruction Set 1
else
Instruction Set 2
be
Tip: Before text file conditional expression [-f "$ file1"] && echo 1 || echo 0
Is equivalent to the bifurcation if [-f "$ file1"]; then echo 1; else echo 0; fi
Multi-branch sentence structure