01 - Shell Basics

Getting started.

There no "right" way, there are some better solutions and worse but there are many ways to get the job done.

The shell uses both built-in commands, that are programmed into the shell program, and then other commands in the path.  If you want to be sure that you use a program on the system you may need to specify the whole path to the program.

I/O Redirection and Pipes

The shell uses different "streams" for input and output.  There are 3 default streams I/O, and they are numbered

0 standard input

1 standard output

2 standard error

You can redirect this output to a number of different places.

If you want to discard the output from a command you can pipe the output to dev null (the system bit bucket)

ls -al > /dev/null

You can use pipe to send the output from one command to another

ls -al /usr/bin | more

That will take the output from ls -al and pipe it into more so it will be paged and you ahd look through the output.

You can also capture output of a command in a variable with

THE_VARIABLE=`date +%Y%m%d`

Control and Exection

For

While

Case

If

Variables

Shell ariables are simple to work with.

You simply set a variable with

The_Shell_Variable="This"

You can print or echo a shell variables with

echo "The Shell Variable is ${The_Shell_Varible}"

There are several predefined variables for use.

PS1 - your pompt

USERNAME - your user name

HOME - your home directory

UID - your user id number

Shell enviornment

These commands display the enviornment

env

set

Different shell variables can effect how the shell and program act.