IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 26 Exercise

Log in to pe15.cs.umb.edu

Run the ssh client

Connect to pe15.cs.umb.edu.

Directory for this Exercise

Go to your ex directory
```
cd  ~/it244/ex
```
Create an ex26 directory
```
mkdir  ex26
```
Go to your ex26 directory
```
cd  ex26
```

Braces with Variables

Define the variable foo
```
foo=FOO
```
Display the value of foo
```
echo $foo
```
Use foo with a period
```
echo $foo.bar
```
Use foo with a slash
```
echo $foo/bar
```
Use foo concatenated with another string
```
echo $foobar
```
bash does not see the variable foo followed by the string "bar".

It sees a new variable, foobar.
Use foo concatenated with another string, but using braces
```
echo ${foo}bar
```

Special Parameters

Look at the running processes
```
ps
```
Use the $$ special parameter
```
echo $$
```
The value returned by $$ is the process ID of your current shell.
Run a command that succeeds
```
ls
```
Use the $? special parameter
```
echo $?
```
The command returns 0 since the previous command executed successfully.
Run a command that fails
```
ls xxxxx
```
Use the $? special parameter
```
echo $?
```
The command returns 2 indicating that the previous command failed.
Run a command in the background
```
sleep 120 &
```
Use the $! special parameter
```
echo $!
```
The number returned is the same as the PID that appeared on the command line when the background job was created.

Positional Parameters

Using nano or an editor of your choice, create the script positionals.sh

Make this script executable
```
chmod 755 positionals.sh
```

Run "positionals.sh" with some arguments

./positionals.sh foo bar bletch

You should see

This shell script was called using ./positionals.sh
This shell script was called using 3 command line arguments
Here are the command line arguments: foo bar bletch
Positional parameter 1: foo
Positional parameter 2: bar
Positional parameter 3: bletch

`shift`

Using nano, or an editor of your choice, create the following shell script shift.sh

Make this script executable
```
chmod 755 shift.sh
```
Run this script with a couple of arguments
```
./shift.sh foo bar bletch
```
You should see
```
foo
bar
bletch
```
The shift command moves each command line argument in turn into the first positional parameter, allowing us to print each argument until there are no more command line arguments left.

`set`

Using nano, or an editor of your choice, create the following shell script set.sh

Make this script executable
```
chmod 755 set.sh
```
Run this script with no arguments
```
./set.sh
```
You should see
```
Here are the arguments given to this script: 
After calling set
Positional parameter 1: foo
Positional parameter 2: bar
Positional parameter 3: bletch
```
Even though no arguments were given to the script at the command line, set created these arguments from within the script.