Shell programming in Linux (i)

Conditional judgment, control structure:

Single Branch if statement

if judgment condition; Then

Statement1

Statement2

...

Fi

Two-branch if statement:

if judgment condition; Then

Statement1

Statement2

...

Else

Statement3

Statement4

...

Fi

Multi-Branched if statement:

if judgment condition 1; Then

Statement1

...

Elif judgment Condition 2; Then

Statement2

...

Elif judgment Condition 3; Then

Statement3

...

Else

Statement4

...

Fi

Test method:

• [Expression]
• [[Expression]]
• Test expression

There are three common conditions tested in bash:

Integer test:

• -EQ: Tests whether two integers are equal, such as $A-eq $B
• -ne: Test whether the two integers are unequal, unequal, true, equal, false;
• -GT: Tests whether one number is greater than the other, greater than, true, or false;
• -LT: Tests whether one number is less than the other, less than, true; otherwise, false;
• -ge: greater than or equal to
• -le: Less than or equal to

Example:
int1=63
int2=77
[$INT 1-eq $INI 2]
[[$INT 1-eq $INT 2]]
Test $INT 1-eq $INT 2

File test:

• -e file: Test files exist
• -F File: Test files are normal files
• -D FILE: Tests whether the specified path is a directory
• -R File: Tests whether the current user has read access to the specified file;
• -W
• -X

Example: [-e/etc/inittab] [-x/etc/rc.d/rc.sysinit]

How to perform arithmetic operations in the shell:
A=3
B=6
1. Let arithmetic operation expression
Let c= $A + $B
2. $[arithmetic operation expression]
c=$[$A + $B]
3, $ (arithmetic operation expression)
c=$ (($A + $B))
4, expr arithmetic expression, the expression in the operands and operators to have a space between, and to use the command reference
c= ' expr $A + $B '

Defining script Exit Status codes

Exit: Exit Script Exit # If the script does not explicitly define exit status code, then the exit code of the last command executed is the exit status code of the script;

Test script for syntax error: bash-n script

Bash-x Script: Stepping

Types of Bash variables:

Local variables (local variables)

Environment variables

Positional variables: $, $, ... shift

Special variables:

• $?
• $#: Number of parameters
• $*: Parameter list
• [email protected]: parameter list

Shell programming in Linux (i)