source : http://linuxcommand.org/lc3_writing_shell_scripts.php
Writing shell scripts
a shell script is file containing a series of commands. The shell reads this file and carries out the commands as though they have been entered directly on the command line.
a shell script is a file that contains ascii text.
to create a shell script, you use a text editor.
Use vi to use text editor, make sure to designate a folder name after typing the vi command
#!/bin/bash
# My first script
echo "Hello World!"
And then… set permission as it follows,
Chmod 755 hello_world
You could ./hello_world, but setting a path where all the files are stored in would be much more easily accessible. For example, all the commands are stored at $PATH. You could add your own path directory by the following
Export PATH=$PATH:directory
Now just typing hello_world would run your script.
A. Editing the scripts you already have
there are two kinds of shell session.
a login shell session is one that prompts for username and password.
a non-login shell session typically occurs for gui.
~/.bashrc = a user's personal start up file. could make your own alias scripts here and you can use the commands regularly.
access the .bashrc file by:
vi .bashrc
and add the following
today() {
echo -n "Today's date is: "
date + "%A, %B %-d, %Y"
}
and restart your terminal to take effect and type today will output :
B. Here Scripts
Create a HTML using script
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#!/bin/bash
# sysinfo_page - A script to produce an HTML file
cat <<- _EOF_
<html>
<head>
<title>
The title of your page
</title>
</head>
<body>
Your page content goes here.
</body>
</html>
_EOF_
http://colorscripter.com/info#e" target="_blank" style="color:#e5e5e5text-decoration:none">Colored by Color Scripter
|
http://colorscripter.com/info#e" target="_blank" style="text-decoration:none;color:white">cs |
and then run:
sysinfo_page > sysinfo_page.html
C. Variables
How to create variables :
variables can be declared on top of shell script as global variables and be used be adding $variablename
variablename="your text here"
there should be no spaces!
variable that comes with the system are called environment variables. environment variables can be checked by typing printenv.
by convention, environment variables are uppercase
D. Command Substitution and Constants
$() tells the shell to substitute the results of the enclosed command.
There is an alternate syntax for $(), which is backticks ``. however ``backticks are older form and for readability, $() is much more prefered.
right_now=$(date +"%x %r %Z")
this is also possible
D.Shell Functions
For maintains and readability, it is often useful to break a single, large task into a series of smaller tasks.
Process of identifying the top-level steps and developing increasingly detailed views of those steps is called top-down design.
When you are developing a program, it is is often a good practice to add a small amount of code, run the script, add some more code, run the script, and so on. This way, if you introduce a mistake into your code, it will be easier to find and correct.
stubbing = havent developed the function but instead of leaving it blank, could add a basic command like echo to run the script.
E. Flow Control - Part 1
The if statement has the following syntax:
if commands; then
commands
[elif commands; then
commands...]
[else
commands]
fi
|
Expression |
Description |
|
-d file |
True if file is a directory. |
|
-e file |
True if file exists. |
|
-f file |
True if file exists and is a regular file. |
|
-L file |
True if file is a symbolic link. |
|
-r file |
True if file is a file readable by you. |
|
-w file |
True if file is a file writable by you. |
|
-x file |
True if file is a file executable by you. |
|
file1 -nt file2 |
True if file1 is newer than (according to modification time) file2 |
|
file1 -ot file2 |
True if file1 is older than file2 |
|
-z string |
True if string is empty. |
|
-n string |
True if string is not empty. |
|
string1 = string2 |
True if string1 equals string2. |
|
string1 != string2 |
True if string1 does not equal string2. |
**Commands issue a value to the system when they terminate, called an exit status.
integer range from 0 to 255,
An 8-bit unsigned integer has a range of 0 to 255, while an 8-bit signed integer has a range of -128 to 127 - both representing 256 distinct numbers.
as seen here, success command always outputs 0. however unsuccesful command results 2.
output nothing if it is false. just like all other programming languages, the basic set of rules are applied.
id -u displays current users id. if your are a superuser the number will be 0.
>&2 is another I/O direction like cat << _EOF_. >&2 redirects the error message to standar error. this is used to separate error message and normal output.
F. Stay out of trouble
when assigning value to a variable, empty value could be placed.
number=
but when including it to the if statement, you must include "" quotation marks around $number or empty value will be read as an equal sign, causing the script the not work properly.
add "-x" beginning of your script file to see your script run on terminal.
G. Keyboard Input and Arithmetic
read command takes input from keyboard.
#!/bin/bash
echo -n "Enter some text > "
read text
echo "You entered: $text"
H. Flow Control - Part 2
The case command has the following form:
case word in
patterns ) commands ;;
esac
looping
#!/bin/bash
number=0
while [ "$number" -lt 10 ]; do
echo "Number = $number"
number=$((number + 1))
done
I. Positional Parameters
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