Shell Programming | Shell Fundamentals

Aug 24 2018 Linux shell 4 minutes read (About 585 words)

shell is a script language and belongs to interpreted language

Variables

One big difference between Shellscript and other programming languages is that there must be no spaces between “=” sign. Such a design has its reason, as shell is not only a programming language, but also a command line interpreter. If you use VAR = value, it’s like being interpreted as running some command called “VAR” and two arguments named “=” and “value”. Therefore that is impossible to run successfully.

Shellscript uses $VAR to reference a variable that has been defined before.

Scope of variables

“local” variable

When running a script file, Linux creates a new process with a unique ‘pid’ for it. We can only use variables created in the script file in this process. Once the process is ending or we are in another process, we cannot use the variables.

“global” variable

However, like the concept of ‘environmental variables’ in Windows, Linux also has global-scope variables by using the source commands or . command for sourcing a script file. This is how the configuration files like .bashrc or .vimrc works (source .bashrc every time before opening a bash and source .vimrc whenever before opening a vim program).

the priority

Think such an occasionally case, if a local defined variable has the same name as a global variable, the local one will have priority. If the local one doesn’t exist or hasn’t been defined yet, the interpreter will use the global ones. If none of them exist, a ‘null’ will be given out.

Statements

if语句

if [ 布尔表达式 ]
then
	若干语句
else
	若干语句
fi

case 语句

case 变量 in
    模式1 ) 若干语句;;
    模式2 ) 若干语句;;
esac

循环语句

循环语句有多种写法

for i in {1..10}
do
    若干语句
done

Special characters

$$$$ indicates the process id(PID) that running the script.

The use of curly braces “{}” can enclose a variable into it even if characters follow with no spaces. See the example below.

#!/bin/sh
echo "What is your name?"
read USER_NAME
echo "Hello $USER_NAME"
echo "I will create you a file called ${USER_NAME}_file"
touch "${USER_NAME}_file"

There is one more advantage that if we type two separated words, the curly braces will prevent touch creating two new files for it.

Comment

Shell uses # to comment a whole line.

Running the script

Write scripts into a readable and executable file. The first line can indicate which program(/bin/sh) to run it. If you are using GNU/Linux, /bin/sh is normally a symbolic link to bash.

#!/bin/sh

Functions

buit-in commands

echo: takes one or more parameters separated by blank characters(spaces or tabs).

expr: Print the value of EXPRESSION to standard output. It supports logical expressions. It can be also used as a simple calculator for integers in shell. But caution that the asteroid * is regarded as a wildcard. So when you do multiplication, you should use \*.

read: reads a line from standard input into the variable supplied.

touch: create a empty file by the name

test: can be used to detect certain files or related properties, and the judgment symbol [] is equivalent.
`test -n string: returns true is the string’s length is over 0

For more Linux builtin commands, go to this article.

#Linux #shell