Bash script programming for

Bash Script Programming:

If statement, Bash-n, Bash-x

CONDITION:
Bash command:
The execution status result of the command;
Success: True
Failure: flase

The meaning of success or failure: Depending on the command used;

Single branch:
if CONDITION; Then
If-true
Fi

Dual Branch:
if CONDITION; Then
If-true
Else
If-false
Fi

Multi-branch:
if CONDITION1; Then
If-true
Elif CONDITION2; Then
If-ture
Elif CONDITION3; Then
If-ture
...
Esle
All-false
Fi

By the condition of judgment, the first encounter as a "true" condition, the execution of its branches, and then end;

Example: User type file path, script to determine file type;
#!/bin/bash
#

Read-p "Enter a file path:" FileName

If [-Z "$filename"]; Then
echo "Usage:enter a file path."
Exit 2
Fi

if [!-e $filename]; Then
echo "No such file."
Exit 3
Fi

If [-f $filename]; Then
echo "A common file."
elif [-D $filename]; Then
echo "A directory."
elif [-l $filename]; Then
echo "A symbolic file."
Else
echo "Other type."
Fi

Note: If statements can be nested;

Loops: For, while, until
Loop body: The code to execute, possibly N times;
Entry conditions:
Exit Criteria:

For loop:
For variable name in list; Do
Loop body
Done

Implementation mechanism:
Assigns the element in the list to the "variable name" in turn; The loop body is executed once each assignment; Until the elements in the list are exhausted, the loop ends;

Example: Add 10 users, user1-user10; password with user name;
#!/bin/bash
#

if [! $UID-eq 0]; Then
echo "Only root."
Exit 1
Fi

For i in {1..10}; Do
If ID user$i &>/dev/null; Then
echo "user$i exists."
Else
Useradd user$i
If [$?-eq 0]; Then
echo "User$i" | passwd--stdin user$i &>/dev/null
echo "Add User$i finished."
Fi
Fi
Done

List Generation Method:
(1) give the list directly;
(2) List of integers:
(a) {start: End
(b) $ (SEQ [start [step]] end)
(3) command to return the list;
$ (COMMAND)
(4) Glob
(b) Variable references;
[email protected], $*


Example: Determine the type of all files under a path
#!/bin/bash
#

For file in $ (Ls/var); Do
If [-f/var/$file]; Then
echo "Common file."
elif [-l/var/$file]; Then
echo "Symbolic file."
elif [-d/var/$file]; Then
echo "Directory."
Else
echo "Other type."
Fi
Done

Example:
#!/bin/bash
#
Declare-i estab=0
Declare-i listen=0
Declare-i other=0

For state in $ (Netstat-tan | grep "^tcp\>" | awk ' {print $NF} '); Do
If ["$state" = = ' established ']; Then
Let estab++
elif ["$state" = = ' LISTEN ']; Then
Let listen++
Else
Let other++
Fi
Done

echo "Established: $estab"
echo "LISTEN: $listen"
echo "Unkown: $other"

In the practice 1:/ETC/RC.D/RC3.D directory, there are several files starting with K and beginning with S, respectively;
Read each file separately, the output of the file starting with K is the file plus stop, and the file output with s begins with the filename plus start;
"K34filename Stop"
"S66filename Start"

Exercise 2: Write a script that uses the ping command to detect the online status of the host between the 172.16.250.1-254;

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Bash script programming for