IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 18 Exercise

More practice with backslash

echo  The University  \[Enter]
of Massachusetts  \[Enter]
at Boston  [Enter]

Use three echo commands on the same command line
```
echo  foo;  echo  bar;  echo  bletch
```
Each echo command generates its own line of output.

Use three more echo commands on the same line

echo  Unix;  echo is not;  echo  for the faint of heart

Use different commands on the same line

echo;  echo  This is what is in my home directory:;  ls  -al ~

Copy hi_bye.sh from ~ghoffman/course_files/it244_files

cp  ~ghoffman/course_files/it244_files/hi_bye.sh  .

Display the contents of hi_bye.sh
```
cat hi_bye.sh
```
Run multiple instances of hi_bye.sh with & as a separator, then run the jobs command
```
./hi_bye.sh  & ./hi_bye.sh  & jobs
```
The ampersand causes each of the two invocations of hi_bye.sh to run in the background.

This means the ampersand has two functions, to cause a command to run in the background and to separate one command from the next.

Notice that jobs is able to report on the two invocations of hi_bye.sh running in the background, because it is running in the foreground.
Wait for completion of jobs

After the second Goodbye appears, hit Enter.

You should see a report that the two jobs have terminated.

Create a local variable
```
greeting=Hello
```
Display the local variable you just created
```
echo  $greeting
```
Assign a string with a space in it to this local variable
```
greeting='Hello there'
```
Display the new value of this variable
```
echo  $greeting
```

Copy print_foo.sh from ~ghoffman/course_files/it244_files

cp  ~ghoffman/course_files/it244_files/print_foo.sh  .

Print this script to the screen
```
cat  print_foo.sh
```
This script simply prints the value of the variable foo to Standard Output.
Create the local variable foo
```
foo=FOO
```
Print the value of foo
```
echo $foo
```
Run print_foo.sh
```
./print_foo.sh
```
The script prints no value for foo, since foo was a local variable defined in your original shell.

The script runs in a subshell and cannot see the local variables of its parent shell.

Set foo to a new value and export it
```
export foo=BAR
```
Print foo in your current shell
```
echo $foo
```
Run print_foo.sh
```
./print_foo.sh
```
Since the export command makes foo a global variable, print_foo.sh can now see this variable in the subshell in which it is run.

Go to ~ghoffman/course_files/it244_files/dir_tree
```
cd  ~ghoffman/course_files/it244_files/dir_tree
```
Look at your current location
```
pwd
```
Look at the contents of the directory
```
ls
```
Use dirs to look at the directory stack
```
dirs
```
It only lists the current directory.
Go down one level using pushd
```
pushd  proj
```
Two directories are now listed.
Show your location
```
pwd
```
Look at the contents of the directory
```
ls
```
Go down another level
```
pushd  proj1
```
Three directories are now listed.
Show your location
```
pwd
```
Go up one level using popd
```
popd
```
Now, only two directories are listed, because you poped the previous directory from the stack.
Show your location
```
pwd
```
Go up one level, suppress output and get location, all on one line
```
popd  >  /dev/null;  pwd
```
By redirecting output to /dev/null, you have stopped popd from printing out the directory stack.

By using a semi-colon followed by pwd, you have displayed your current location without having to wait for a prompt.

Go back to your ex18 directory
```
cd  ~/it244/ex/ex18
```
Create the script ex18.sh
```
nano  ex18.sh
```
Write into this file commands from certain of the sections above

Include Unix commands from the section Important Keyword Shell variables, up to and including the section The Directory Stack.
Save this file

Save and quit.
Make this script executable
```
chmod 755 ex18.sh
```
Run this script looking for error messages
```
bash  ex18.sh > /dev/null
```
Running ex18.sh this way will only print error messages.

If you see nothing your script has no Unix errors.

If you find any errors, fix them.
Test your script
```
~ghoffman/it244_test/ex18.sh
```
When the script asks if you are ready for more, hit Return or Enter.
Let me check your script