• input and output,user input
• if
• exit statues
• functions
• wildcards
• for loops
• case statements
• logging
• debugging tips
• bash shell options
• more

---

• download
  shell-script-course/debugging/lessons
  shell-script-course/debugging/practice-exercies

• learn
  1 use variable
  2 perform tests and make decisions
  3 accept command line arguments
  4 accept input from a user

demo

    #!/bin/bash 
    echo "scripting is fun!" 
    --------------------------    
    chmod 755 script.sh 
    ./script.sh

not just shell script

    #!/usr/bin/python 
    print "this is a python script" 
    ------------------------------
    chmod 755 hi.py 
    ./hi.py

Variables

• storage locations that have a name
• name-value pairs
• uppercase,case sensitive
• VARIABLE_NAME="Value"

Variable Usage

    #!/bin/bash 
    MY_SHELL="bash" 
    echo "I like the $MY_SHELL shell."

    #!/bin/bash 
    MY_SHELL="bash"
    echo "I like the ${MY_SHELL} shell."

    #!/bin/bash
    MY_SHELL="bash"
    echo "I am ${MY_SHELL}ing on my keyboard."
    
    OUTPUT:
    I am bashing on my keyboard.

    #!/bin/bash
    MY_SHELL="bash"
    echo "I am $MY_SHELLing on my keyboard."
    
    OUTPUT:
    I am  on my keyboard.

    #!/bin/bash
    SERVER_NAME=$(hostname)
    echo "you are running this script on ${SERVER_NAME}."
    
    OUTPUT:
    you are running this script on linuxsvr.

    #!/bin/bash
    SERVER_NAME=`hostname`
    echo "you are running this script on ${SERVER_NAME}."
    
    OUTPUT:
    you are running this script on linuxsvr.

Variable Names（等号前后不能有空格）

• valid:

  • FIRST3LETTERS="ABC"
  • FIRST_THREE_LETTERS="ABC"
  • firstThreeLetters="ABC"

• Invalid:

  • 3LETTERS="ABC"
  • first-three-letters="ABC"
  • first@Three@Letters="ABC"

Tests：success:0 failed:1

• Syntax:
  • [ condition-to-test-for ]
• Example:
  • [ -e /etc/passwd ]

File operators(tests)

    -d FILE     ture if file is directory.
    -e FILE     true if file exists.   
    -f FILE     true if file exists and is a regular file
    -r FILE     true if file is readable by you
    -s FILE     true if file exists and is not empty.
    -w FILE     true if file is writable to you.
    -x FILE     true is file is executable by you.

String operators(tests)

    -z STRING   true if string is empty.
    -n STRING   true if string is not empty.
    STRING1=STRING2     true is the strings are equal.
    STRING1!=STRING2    true is the strings are not equal.

Arithmetic operators(tests)

    arg1 -eq arg2
    arg1 -ne arg2
    
    arg1 -lt arg2
    arg1 -le arg2
    
    arg1 -gt arg2
    arg1 -ge arg2

Making Decisions - The if statement

    if [ condiction-is-true ]
    then
        command 1
        command 2
        command N
    fi

    #!/bin/bash
    MY_SHELL="bash"
    if [ "$MY_SHELL" = "bash" ]
    then
        echo "you seen to like the bash shell."
    fi
    
    OUTPUT:
    you seen to like the bash shell.

if/else

    if [ condition-is-true ]
    then
        command N
    else
        command N
    fi

    #!/bin/bash
    MY_SHELL="csh"
    if [ "$MY_SHELL" = "bash" ]
    then
        echo "you seem to like bash shell."
    else
        echo "you don't seem to like bash shell."
    fi

if/elif/else

    if [ condiction-is-true ]
    then
        command N
    elif [ condition-is-true ]
    then
        command N
    else 
        command N
    fi  

    #!/bin/bash
    MY_SHELL="csh"
    if [ "$MY_SHELL" = "bash" ]
    then
        echo "you seem to like bash shell."
    elif [ "$MY_SHELL" = "csh" ]
    then
        echo "you don't seem to like csh shell."
    else
        echo "you don't seem to like csh or bash shells."
    fi

For loop

    for VARIABLE_NAME in ITEM1 ITEM_N
    do
        command 1
        command 2
        command N
    done    

    #!/bin/bash
    for COLOR in red green blue
    do
        echo "COLOR: $COLOR"
    done
   OUTPUT:
   COLOR: red
   COLOR: green
   COLOR: blue

    #!/bin/bash
    COLORS="red green blue"
    
    for COLOR in $COLORS
    do
        echo "COLOR: $COLOR"
    done
   OUTPUT:
   COLOR: red
   COLOR: green
   COLOR: blue

    #!/bin/bash
    PICTURES=$(ls *jpg)
    DATE=$(date +%F)
    for PICTURE in $PICTURES
    do
        echo "Renaming ${PICTURE} to ${DATE}-${PICTURE}"
        mv ${PICTURE} ${DATE}-${PICTURE}
    done
    
    OUTPUT:
    $ ls
    bear.jpg
    $ ./rename-pics.sh
    Renaming bear.jpg to 2015-03-06-bear.jpg
    $ ls
    2015-03-06-bear.jpg 

parameters 9

• todo

Exit statuses

• how to check the exit status of a command
• how to make decisions based on the status
• how to use exit statuses in your own scripts

Exit Status / Return Code

• Every command returns an exit status
• Range from 0 to 255
• 0 = success
• Other than 0 = error condition
• Use for error checking
• Use man or info to find meaning of exit status

Checking the exit status

• $? contains the return code of the previously executed command.

    ls /not/here
    echo "$?"
    
    OUTPUT:
    2

    HOST="google.com"
    ping -c l $HOST
    if [ "$?" -eq "0" ]
    then
        echo "$HOST reachable."
    else
        echo "$HOST unreachable."
    fi     

    HOST="google.com"
    ping -c l $HOST
    if [ "$?" -nq "0" ]
    then
        echo "$HOST unreachable."
    fi     

    HOST="google.com"
    ping -c l $HOST
    RETURN_CODE=$?
    
    if [ "$RETURN_CODE" -ne "0" ]
    then
        echo "$HOST unreachable."
    fi     

&& and ||

• && = AND (前一个是true，后一个才会执行)

    mkdir /tmp/bak && cp test.txt /tmp/bak/

• || = OR (前一个是false，后一个才会执行)

    cp test.txt /tmp/bak/ || cp test.txt /tmp

    #!/bin/bash
    HOST="google.com"
    ping -c l $HOST && echo "$HOST reachable."

    #!/bin/bash
    HOST="google.com"
    ping -c l $HOST || echo "$HOST unreachable."

The semicolon ;

• separate commands with a semicolon to ensure they all get executed.
• 和前一个命令是否成功无关，命令都会被执行，和换行是一个意思

    cp test.txt /tmp/bk/ ; cp test.txt /tmp
    #Same as:
    cp test.txt /tmp/bk/
    cp test.txt /tmp

Exit command

• Explicitly define the return code
  • exit 0 -- 255
• the default value is that of the last command executed
• 脚本的任何地方都可以使用exit命令

    HOST="google.com"
    ping -v 1 $HOST
    if [ "$?" -ne "0" ]
    then
        echo "$HOST unreachable."
        exit 1
    fi
    exit 0

Summary

• all command return an exit status
• 0 - 255
• 0 = success
• other than 0 = error condition
• $? contains the exit status
• Decision making -if , &&, ||
• exit

Functions

• why to use functions
• how to create them
• how to use them
• variable scope
• function parameters
• exit statuses and return codes

Functions

• if you're repeating yourself ,use a function.
• reusable code
• must be defined before use
• has parameter support

    function function-name(){
        #code goes here
      }
    
    function-name(){
        #code goes here
    }
    
    #!/bin/bash
    function hello(){
        echo "Hello!"
    }
    hello

    #!/bin/bash
    function hello(){
        echo "hello！"
        now
    }
    function now(){
        echo "It's $(date +%r)"
    }
    hello

    #!/bin/bash
    function hello(){
        echo "hello！"
        now
    }
    hello # 会报错的
    function now(){
        echo "It's $(date +%r)"
    }

Positional Parameters

• functions can accept parameters
• the first parameter is stored in $1
• the second parameter is stored in $2, etc.
• $@ contains all of the parameters
• just like shell scripts.
  • $0 = the script itself, not function name

    #!/bin/bash
    function hello(){
        echo "Hello $1"
    }
    hello Jason
    
    # OUTPUT:
    # Hello Jason

    #!/bin/bash
    function hello(){
        for NAME in $@
        do  
            echo "hello $NAME"
        done
    }
    
    hello jason dan ryan
    # OUTPUT:
    # Hello jason
    # Hello dan
    # Hello v    

variable scope

• by default, variables are global
• variable have to be defined before used.

    GLOBAL_VAR=1
    # GLOBAL_VAL is available
    # in the function.
    my_function

    # GLOBAL_VAL is NOT available
    # in the function.
    my_function
    GLOBAL_VAR=1

    #!/bin/bash
    my_function(){
       GLOBAL_VAR=1 
    }
    
    # GLOBAL_VAL is NOT available yet
    echo $GLOBAL_VAR
    my_function
    # GLOBAL_VAL is NOW available yet
    echo $GLOBAL_VAR

local Variables

• can only be accessed within the function.
• create using the local keyword
  • local LOCAL_VAR=1
• only functions can have local variables
• best practice to keep variables local in functions

Exit Status(Return Code)

• functions have an exit status

• explicitly

  • return <RETURN_CODE>

• Implicity

  • the exit status of the last command executed in the function

• Valid exit codes range from 0- 255

• 0 = success

• $?=the exit status

    my_function
    echo $?

    # todo :.

Summary

• DRY(don't repeat yourself)
• global and local variables
• parameters
• exit statuses

Wildcards

• *-matches zero or more characters

  • *.txt
  • a*
  • a*.txt

• ?-matches exactly one character

  • ?.txt
  • a?
  • a?.txt

More Wildcards-Character Classes

• []- a character class

  • matches any of the characters include between the brackets.
    matches exactly one character.
  • [aeiou]
  • ca[nt]*
    • can
    • cat
    • candy
    • catch

• [!]- matches any of the characters NOT included between the brackets.
  matches exactly one character

  • [!aeiou]*
    • baseball
    • cricket

more wildcards - Ranges

• User two characters separated by hyphen to create range in a
  character class.

• [a-g]*

  • matches all files that start with a,b,c,...g

• [3-6]*

  • matches all files that start with 3,4,5,6

Name character class

• [[:alpha:]] 字母
• [[:alnum:]] 数字字母
• [[:digit:]] 数字
• [[:lower:]] 小写字母
• [[:space:]] 字符
• [[:upper:]] 大写字母

Mathching wildcard patterns

• -escape character. use if you want to match a wildcard
  character
  • match all files that end with a question mark:
    • *?
      • done？

summary

• ?
• []
• [0-3]
• [[:digit:]]

why use wildcards?

• wildcards are great when you want to work on a group of files or directories.

    #!/bin/bash
    cd /var/www
    cp *.html /var/www-just-html

    #!/bin/bash
    cd /var/www
    for FILE in *.html
    do
        echo "copy $FILE"
        cp $FILE /var/www-just-html
    done
    
    OUTPUT:
    copy about.html
    copy content.html
    copy index.html

    #!/bin/bash
    
    for FILE in /var/www/*.html
    do
        echo "copy $FILE"
        cp $FILE /var/www-just-html
    done
    
    OUTPUT:
    copy /var/www/about.html
    copy /var/www/content.html
    copy /var/www/index.html

summary

• just like on the command line.
• in loops
• supply a directory in the wildcard or use the cd command to change the current directory

Case Statements

• alternative to if statements
  • if ["$VAR"="one"]
  • elif ["$VAR"="two"]
  • elif ["$VAR"="three"]
  • elif ["$VAR"="four"]
• may be easier to read than complex if statements

case "$VAR" in
    pattern_1)
        # commands go here
        ;;
    patter_N)
        # commands go here    
        ;;
esac

    case "$1" in
        start)
            /usr/sbin/sshd
            ;;
        stop)
            kill $(cat /var/run/sshd.pid)
            ;;
    esac           

    case "$1" in
        start)
            /usr/sbin/sshd
            ;;
        stop)
            kill $(cat /var/run/sshd.pid)
            ;;
        *)
            echo "Usage: $0 start|stop" ; exit 1
            ;;    
    esac           

    case "$1" in
        start|START)
            /usr/sbin/sshd
            ;;
        stop|STOP)
            kill $(cat /var/run/sshd.pid)
            ;;
        *)
            echo "Usage: $0 start|stop" ; exit 1
            ;;    
    esac           

    read -p "Enter y or n: " ANSWER
    case "$ANSWER" in
        [yY]|[yY][eE][sS])
            echo "your answered yes."
            ;;
        [nN]|[nN][oO])
            echo "your answered no."
            ;;
        *)
            echo "Invalid answer."
        ;;
     esac                   

    read -p "Enter y or n: " ANSWER
    case "$ANSWER" in
        [yY]*)
            echo "your answered yes."
            ;;
        *)
            echo "Invalid answer."
        ;;
     esac                   

summary

• can be used in place of if statements.
• patterns can include wildcards.
• multiple pattern matching using a pipe.

Logging

what you will learn

• why log
• Syslog standard
• Generating log messages
• Custom logging functions

Logging

• Logs are the who ,what , when, where ,and why
• Output may scroll of the screen
• Script may run unattended(via cron, etc)

todo:

summary

• Why log
• syslog standard
• Generating log messages
• Custom logging functions

while loops

• what you will learn
• while loops
• infinite loops
• loop control
  • explicit number of times
  • user input
  • command exit status
• reading files, line-by-line
• break and continue

    while [ condition_is _true ]
    do
        command 1
        command 2
        command N
    done

    while [ condition_is_true ]
    do
        # commands change the condition
        command 1
        command 2
        command N
    done

infinite loops

    while [ condition_is_true ]
    do
        # commands do not change
        # the condition
        command N  
    done