特殊:Badtitle/NS100:Beginners/BashScripting/zh

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翻译人员:

ganthur

Bash 脚本是最容易学习的脚本类型之一,它十分类似于Windows下的批处理脚本. Bash非常的灵活, 而且它有很多你在批处理中无法看到的高级特性. 但如果你对电脑一无所知,那么它对你来说毫无用处. 一般来说,你会喜欢学习Bash,因为大多数在Ubuntu下的日常工作,都可以用终端来解决. 你也会渐渐明白大部分事情都可以通过用户图形界面和命令行来完成, 然而有些事情选择其中一种方法要更容易一些. 例如,使用命令行来更改一个文件夹及其子文件夹中的文件的权限要比使用用户图形界面容易的多.

Intro

In this document we will discuss useful everyday commands, as well as going a bit more in depth into scripting and semi-advanced features of Bash. Bash is not only used to run programs and applications, but it can also be used to write programs or scripts.

Bash -- Everyday Ubuntu life

During your time as an Ubuntu'er you will use the terminal to perform tasks such as

• Creating folders
• Deleting files
• Deleting folders and their sub-folders
• Opening applications as root
• Backing up your files
• Backing up your folders
• Checking system performance
• Check Devices
• Checking wireless connection

And many other things, the list above will be the commands we will discuss.

Commands

Creating folders

Creating folders can be done simply in the file manager nautilus by right clicking and selecting 'Create Folder', but if you want to do this from a cli environment you would type the following in the terminal:

sudo mkdir /home/joe/Desktop/new_folder

the mkdir (make directory) command creates the folder then the file path tells it where to create the folder. (I use sudo because if you try to create a folder anywhere other then /home or one of its sub-directories you will need root/superuser privileges. This goes for all commands in this section.)

Deleting files

Deleting files can be done like this

sudo rm /home/joe/file_to_be_deleted

the rm (remove) command is used to remove anything through a cli environment.

Deleting folders and their sub-folders

The command you are about to read can potentially (if used incorrectly) destroy your system!

sudo rm -rf /home/joe/useless_Parent_folder

This command is slightly different to the one before, it uses two options '-r' which means recursive (will delete the folder and all sub-folders) and '-f' means force (will not ask for your permission). This command is perfectly fine for deleting a dir and all its sub-dirs. The next commands should !!**!!NEVER!!**!! be run. Unless you want to say goodbye to your system.

sudo rm -rf /*
sudo rm -rf /

This will delete everything from your root folder downwards, which if you did a standard install would be everything.

Opening applications as root

Sometimes you will want to edit a config file in your root folder, in order to save changes to this file you need root privileges so we need to open our text editor as root. (assuming your text editor is gedit)

gksudo gedit /path/to/conf_file.txt

gksudo is the same as sudo only it brings a box up to ask for your password, gksudo should always be used to open graphical applications as root. Serious damage can be done to your system by editing these files. It is advised to create a backup of any file you edit.

Backing up your files

To create a backup of a file, we're going to use the cp (copy) command. The basic syntax for cp is as follows:

cp source_file dest_file

This will copy the 'source_file' to the 'dest_file'. Now, using the previous example, we want to backup '/path/to/conf_file.txt'. To accomplish this, we type the following:

sudo cp /path/to/conf_file.txt /path/to/conf_file.txt.old

That's fine and dandy, but what if I want to copy my file to another directory? Well that's just as easy. Let's say instead of copying /path/to/conf_file.txt to the /path/to/ directory, you want to copy it to a directory where you store all of your backup files, say /my/backup/folder/. In order to accomplish this you would type:

sudo cp /path/to/conf_file.txt /my/backup/folder/    #saves conf_file.txt to /my/backup/folder/
#OR
sudo cp /path/to/conf_file.txt /my/backup/folder/conf_file_new_name.txt

• • • This is a typical safety measure that has saved many users in the past from a complete disaster.***

Okay, so we know how to copy a file a) to a different filename and b) to a different folder. But how do we copy entire directories?

Backing up your directories

To backup one directory to another, we introduce cps '-r' (recursive) option. The basic syntax is as follow:

cp -r /directory/to/be/copied/ /where/to/copy/to/

So if we wanted to copy all of the contents of our /path/to/ folder to our /my/backup/folder, we would type the following:

sudo cp -r /path/to/ /my/backup/folder/foldername    #foldername can be whatever you want the foldername to be

Checking system performance

If your computer starts to lag, you can see which applications are using the most CPU power with this command:

top

This is generally the same information given as the GUI application 'System Monitor'.

Check Devices

USB Devices If a USB device stops working, you may want to see if it is still connected/detected. To check if a device is connected/detected, type the following:

lsusb

PCI Devices PCI devices are checked with:

lspci

Checking wireless connection

To check the status on your wireless connection, use:

iwconfig

This also shows packets sent/received.

Scripting

Bash is primarily a scripting language, so it would be a crime not to talk about scripting. Let's dive straight in with a bash script. More precisely the infamous "Hello World" script. You can create a bash script by opening your favorite text-editor to edit your script and then saving it (typically the .sh file extension is used for your reference, but is not necessary. In our examples, we will be using the .sh extension).

 #!/bin/bash         

echo "Hello, World"

The first line of the script just defines which interpreter to use. NOTE: There is no leading whitespace before #!/bin/bash. That's it, simple as that. To run a bash script you first have to have the correct file permissions. We do this with chmod (change mode) as follows:

sudo chmod 700 /where/i/saved/it/hello_world.sh   #Gives read,write,execute permissions to the Owner
# OR
sudo chmod a+x /where/i/saved/it/hello_world.sh   #Gives everyone execute permissions

This will give the file the appropriate permissions so that it can be executed. Now open a terminal and run the script like this:

/where/i/saved/it/hello_world.sh

Hopefully you should have seen it print Hello, World onto your screen. If so well done! That is your first Bash script. TIP If you type:

pwd

You will see the directory that you are currently working in (pwd stands for 'print working directory'). If your current working directory is /where/i/saved/it/, then you can shorten the above command to:

prompt$ pwd
/where/i/saved/it
prompt$ ./hello_world.sh

Now, lets get to more interesting aspects of Bash programming, Variables!

Variables

Variables store information basically. You set variables like this

var="FOO"

'var' can be anything you want as long as it doesn't begin with a number. "FOO" can be anything you want. To access the information from the variable you need to put a '$' in front of it like this:

var="FOO"
echo $var

Try entering those lines into a terminal one at a time; you will see that the first one just gives you another prompt and the second one prints FOO. But that's all a bit boring. So let's make a script to ask the user for some information and then echo that information.

 #!/bin/bash
clear
echo "Please enter your name"
read name
echo "Please enter your age"
read age
echo "Please enter your sex. Male/Female"
read sex
echo "So you're a $age year old $sex called $name"

read allows the user to input information where it is then stored in the variable defined after the read. read variable would take whatever input the user entered and store it in $variable. We then access this with echo and set up a neat sentence. This script is reasonably messy though; read has another function that could halve the size of this script.

#! /bin/bash
clear
read -p "Please enter your name  : " name
read -p "Please enter your age  : " age
read -p "Please enter your sex. Male/Female  : " sex
echo "So you're a $age year old $sex called $name"

That is more efficient code. However it's still a bit messy when run. A solution? Good old white spaces!

#! /bin/bash
clear
read -p "Please enter your name  : " name
echo ""
read -p "Please enter your age  : " age
echo ""
read -p "Please enter your sex. Male/Female  : " sex
echo ""
echo "So you're a $age year old $sex called $name"

Now we have an efficient and clean Bash script.

If Statements

An if statement can be used to check for something and do something else depending on the outcome of the check. For example, if I had an 'apple', I would want to make sure it's still an 'apple' and not an 'orange' because I don't like Oranges! The syntax for an if statement is

if [something]
	then
	elif
	then
	elif
	then
	....etc....
	else
fi

The elif is not necessary, but it can be used if needed. An if statement to check if our $fruit variable is an 'apple' would look like this

if [ $fruit = apple ]
	then echo "Good, I like Apples"
	else echo "Oh no, I hate Oranges!"
fi

Just to explain this statement,

if [ the contents of $fruit is 'apple' ]
	then say "Good, I like Apples"
	if it's not, then say "Oh no, I hate Oranges!"
finish

If statements are an easy concept to grasp as they are similar to the "if" used in spoken English. But say you wanted to have 4 or 5 checks, the answer may be to write 4 or 5 statements but that's not the most practical way. This is where elif comes in handy.

if [ $fruit = apple ]
	then echo "Good, I like Apples"
elif [ $fruit = pear ]
	then echo "Good, I like Pears"
elif [ $fruit = banana ]
	then echo "Good, I like Bananas"
	else echo "Oh no, I hate Oranges!"
fi

This saves us from from repetitive scripting. There are better ways to check what the fruit is, but we won't go into that now.

Variables

Variables store information basically. You set variables like this

var="FOO"

'var' can be anything you want as long as it doesn't begin with a number. "FOO" can be anything you want. To access the information from the variable you need to put a '$' in front of it like this:

var="FOO"
echo $var

Try entering those lines into a terminal one at a time; you will see that the first one just gives you another prompt and the second one prints FOO. But that's all a bit boring. So let's make a script to ask the user for some information and then echo that information.

 #!/bin/bash
clear
echo "Please enter your name"
read name
echo "Please enter your age"
read age
echo "Please enter your sex. Male/Female"
read sex
echo "So you're a $age year old $sex called $name"

read allows the user to input information where it is then stored in the variable defined after the read. read variable would take whatever input the user entered and store it in $variable. We then access this with echo and set up a neat sentence. This script is reasonably messy though; read has another function that could halve the size of this script.

#! /bin/bash
clear
read -p "Please enter your name  : " name
read -p "Please enter your age  : " age
read -p "Please enter your sex. Male/Female  : " sex
echo "So you're a $age year old $sex called $name"

That is more efficient code. However it's still a bit messy when run. A solution? Good old white spaces!

#! /bin/bash
clear
read -p "Please enter your name  : " name
echo ""
read -p "Please enter your age  : " age
echo ""
read -p "Please enter your sex. Male/Female  : " sex
echo ""
echo "So you're a $age year old $sex called $name"

Now we have an efficient and clean Bash script.

If Statements

An if statement can be used to check for something and do something else depending on the outcome of the check. For example, if I had an 'apple', I would want to make sure it's still an 'apple' and not an 'orange' because I don't like Oranges! The syntax for an if statement is

if [something]
	then
	elif
	then
	elif
	then
	....etc....
	else
fi

The elif is not necessary, but it can be used if needed. An if statement to check if our $fruit variable is an 'apple' would look like this

if [ $fruit = apple ]
	then echo "Good, I like Apples"
	else echo "Oh no, I hate Oranges!"
fi

Just to explain this statement,

if [ the contents of $fruit is 'apple' ]
	then say "Good, I like Apples"
	if it's not, then say "Oh no, I hate Oranges!"
finish

If statements are an easy concept to grasp as they are similar to the "if" used in spoken English. But say you wanted to have 4 or 5 checks, the answer may be to write 4 or 5 statements but that's not the most practical way. This is where elif comes in handy.

if [ $fruit = apple ]
	then echo "Good, I like Apples"
elif [ $fruit = pear ]
	then echo "Good, I like Pears"
elif [ $fruit = banana ]
	then echo "Good, I like Bananas"
	else echo "Oh no, I hate Oranges!"
fi

This saves us from from repetitive scripting. There are better ways to check what the fruit is, but we won't go into that now.

Storing application stdout to a variable:

Application stdout ( what you see on the terminal screen, with an un-piped application ) can be saved and used in Bash. The simplest and most elegant way is to use the back-tick (`) this is not an apostrophe or quote (' or ") and is found on the same key as the tilde (~)

Example

#!/bin/bash
fooVar=`who`
echo $fooVar

This code should output the current users, their respective ttys, and date of login. Note that this strips newlines. Be sure to do any parsing in line ( | grep, etc ) and then pass it to a variable. We will try this again, but grep for tty7, the GUI's tty.

Example 2

fooVar=`who | grep tty7`
echo $fooVar

This should output the single user that is currently logged into the WM. Let's move on to more advanced data manipulation within back ticks.

Example

#!/bin/bash
fooVar=`who`
echo $fooVar

This code should output the current users, their respective ttys, and date of login. Note that this strips newlines. Be sure to do any parsing in line ( | grep, etc ) and then pass it to a variable. We will try this again, but grep for tty7, the GUI's tty.

Example 2

fooVar=`who | grep tty7`
echo $fooVar

This should output the single user that is currently logged into the WM. Let's move on to more advanced data manipulation within back ticks.

FUNctions

Bash lets you create a function on the fly, really handy if you plan on using a code block more then once. Functions reduce the amounts of editing you have to do in a script, if and when you have to update your script. Let's get to it!

Example

Here is an example script:

#!/bin/bash

echo "echo is Called"
echo "Functions are FUN!"
echo "echo is Called"

Although this example is simple, you can see that if you want to change "echo is Called" to say "foo is Called" you would have to edit twice. Below is the same app using functions.

#!/bin/bash
echoFunction() {
  echo "echo is Called"
}
fooBar() {
  echo "Functions are FUN!"
}

echoFunction;
fooBar;
echoFunction;
# You call functions without (), just the function name then a semicolon.

This example, as you can see may be longer now, but you can imagine how, adding features, this will eliminate code and reduce complexity. Also, you can see if you want to change the echo call, you have to edit one line, not two.

Debugging

I always find it useful to trace a script to find out why something does not work as expected. To trace, start it trough bash explicitly and use the -x option, like so:

bash -x ./script.sh

This will write each command to standard error (preceded by a ‘+ ’) before it is executed.

Example

Here is an example script:

#!/bin/bash

echo "echo is Called"
echo "Functions are FUN!"
echo "echo is Called"

Although this example is simple, you can see that if you want to change "echo is Called" to say "foo is Called" you would have to edit twice. Below is the same app using functions.

#!/bin/bash
echoFunction() {
  echo "echo is Called"
}
fooBar() {
  echo "Functions are FUN!"
}

echoFunction;
fooBar;
echoFunction;
# You call functions without (), just the function name then a semicolon.

This example, as you can see may be longer now, but you can imagine how, adding features, this will eliminate code and reduce complexity. Also, you can see if you want to change the echo call, you have to edit one line, not two.

Debugging

I always find it useful to trace a script to find out why something does not work as expected. To trace, start it trough bash explicitly and use the -x option, like so:

bash -x ./script.sh

This will write each command to standard error (preceded by a ‘+ ’) before it is executed.

Debugging

I always find it useful to trace a script to find out why something does not work as expected. To trace, start it trough bash explicitly and use the -x option, like so:

bash -x ./script.sh

This will write each command to standard error (preceded by a ‘+ ’) before it is executed.

Other Scripting Languages related to Bash

tr

tr is one of the most basic applications to pipe data through that uses a basic scripting syntax. In this case, it accepts Regular Expressions. Let's do a normally complicated task, transforming a string to all uppercase.

Example

#!/bin/bash
read foo
var=`echo $foo | tr "{a-z}" "{A-Z}"`
 # {a-z} Matches a through z
 # {A-Z} matches A through Z
echo $var

The output should look something like this:

this is a test
THIS IS A TEST

tr also can TRanslate strings, so let's translate all "tar" in $foo to "bar".

Example

#!/bin/bash
read foo
var=`echo $foo | tr "t" "b"`

the output should look something like this:

I love tars
I love bars

awk

AWK ( Short for Aho, Weinberger & Kernighan ) awk has its own custom scripting language, suitable for a tutorial by itself, so I will cover only the basics to help assist when you are bash scripting. This is not meant to be complete or comprehensive in any way.

pidof clone

Let's make a quick pidof clone that prompts for a process identifier, then echoes the process ID.

#!/bin/bash
read pname
ps -ef | grep -v grep | grep $pname | awk '{print $2}'

Single quotes are used to pass the awk command(s). The curly braces are to use the awk language (for stuff like prints, ifs, etc.). Print prints the column passed given by the $ markup, space delimited. There are a lot more commands than the print command, including if statements, etc., and is worth looking into if you are interested in what you see here!

sed

sed is one of the most complicated scripting languages on the GNU / Linux system. I am only going to cover the s/ command here.

Basic Substitution

Try this out to show that sed can not only replace inline, but unlike tr, replace with a longer or shorter string than before.

#!/bin/bash
read foo
echo $foo | sed "s/foo/bars/"

When this command is run, it should substitute the first appearance of "foo" with "bars". This is an example of the output.

I love to go to foo
I love to go to bars

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