Shell Script Advanced

Shell script advanced One, Shell script advanced condition test

When you write a shell script, you often need to judge some conditions, and you can use Test command test to help complete the test process. The test command tests whether a particular expression is true, and when the condition is true, the return value after the command executes is 0, otherwise it is a different value.

The basic format for conditional testing is:

Format 1
Test EXPRESSION
Format 2
[EXPRESSION]
Note: When using square brackets, spaces must be added to both sides of the condition.

Common types of tests are

• File test
• Numerical test
• String test
• Combination Test
• File test

File presence and category testing

-a FILE: Same-E
-e File: Test for existence of files, existence is true, otherwise false
-B File: Exists and is a block device file
-C file: exists and is a character device file
-D file: Exists and is a catalog file
-F file: exists and is a normal file
-H file or-L file: Existing and Symbolic link files
-P file: exists and is a named pipe file
-S file: exists and is a socket file

File Permission test

-R FILE: exists and is readable
-W FILE: exists and is writable
-X FILE: exists and is executable
-U FILE: Exists and has suid permissions
-G FILE: Exists and has Sgid permissions
-K FILE: Exists and has sticky permissions

File Properties Test

S FILE: exists and is not empty
-T FD:FD indicates whether the file descriptor is open and related to a terminal
-N File: whether the files have been modified since the last time they were read
-O File: Whether the current active user is a file owner
-G file: whether the current active user is a group of files
File1-ef whether File2:file1 and FILE2 point to the same inode on the same device
File1-nt File2:file1 is new to FILE2 (Mtime)
File1-ot File2:file1 is older than FILE2

Numerical test

-GT is greater than
-ge is greater than or equal to
-eq is equal to
-ne is not equal to
-lt is less than
-le is less than or equal to

String test

= = is equal to
is greater than (ASCII code)
< is less than
! = is not equal to
Whether the =~ can be matched by the pattern on the right (typically used in [[]])
-Z verifies whether the string is empty, empty is true, and non-empty is false
-O and-Z opposite

Combination Test

The first way:
COMMAND1 && COMMAND2 and
COMMAND1 | | COMMAND2 or
! COMMAND Non-
The second way:
Expression1-a EXPRESSION2 and
Expression1-o EXPRESSION2 or
! EXPRESSION Non-

Second, the Shell script advanced condition statement

? ? The shell has its own process Control statement, which includes conditional statements, loop statements, and so on.

Conditional Select if statement

Single branch:

if judgment condition; then
Branch code with true condition
Fi
Example:

#!/bin/bash
Use=df -hT | grep "/boot" | awk ‘{print $6}‘ | cut -d "%" -f1
If [$use-gt];then
echo "warning!! /boot disk is full "
Fi

Dual Branch:

if judgment condition; Then
Branch code with true condition
Else
The condition is a false branch code
Fi

Example:

If ping-c1-w2 station1 &>/dev/null; Then
Echo ' Station1 is up '
Else
Echo ' Station1 is unexpectedly down! '
Exit 1
Fi

Multi-branch:

if judgment condition 1; Then
Branch code with true condition
Elif judgment Condition 2; Then
Branch code with true condition
Elif judgment Condition 3; Then
Branch code with true condition
Else
The above conditions are false branch codes
Fi

Example:

#!/bin/sh
Scores=40
if [[$scores-GT 90]]; Then
echo "Very good!"
elif [[$scores-GT 80]]; Then
echo "good!"
elif [[$scores-GT 60]]; Then
echo "Pass!"
Else
echo "No pass!"
Fi

Conditional Judgment Case Statement

Case variable reference in
PAT1)
Branch 1
;;
PAT2)
Branch 2
;;
...
*)
Default Branch
;;
Esac

Example:

#!/bin/sh
Case $ in
Start | Begin
echo "Start Something"
;;
Stop | End
echo "Stop Something"
;;
*)
echo "Ignorant"
;;
Esac

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Shell Script Advanced