Basic Excercises

2. Basic Tasks

2.1. I/O Redirection
2.1.1. Task — Hello World

Write a shell script greeting.sh to output the message message Hello World to standard output.

A shell script is simply a text file containing a sequence of shell commands.

Run your script thus:

$ sh greeting.sh
2.1.2. Task — Create message Hello World in greeting.txt

Use I/O redirection and re-run your script such that its output is redirected to the file greeting.txt.

After running your script, you can view the contents of greeting.txt with the command:

$ cat greeting.txt
2.1.3. Task — Multiple Lines

Write a shell script rhyme.sh which creates a file rhyme.txt containing the following lines:

Mary had a little lamb
Its fleece was white as snow.
And everywhere that Mary went
the lamb was sure to go.

Your script should use only one I/O redirection operator

Your script may not involve any external commands.

2.1.4. Task — Line Numbers

You should now have a file rhyme.txt.

Use the cat command to output the contents of that file in the format:

     1 Mary had a little lamb
     2 Its fleece was white as snow.
     3 And everywhere that Mary went
     4 the lamb was sure to go.
Tip
Read the cat manual page.
2.1.5. Task — Standard Output and Standard Error

Write a shell script named outerr.sh which outputs the message:

I am standard output
to standard output, and the message:

I am standard error
to standard error.

Demonstrate that your script functions correctly:

first by redirecting standard output to /dev/null,

then by redirecting standard error to /dev/null.

2.1.6. Task — script.sh

Write a shell script script.sh which is called as:

$ sh script.sh FILE COMMAND [ARGUMENTS...]
Your script should run the indicated command (with its arguments, if any) leaving all of its standard and error output in the indicated file.

2.2. The read command
2.2.1. Task — head.sh

Write a shell script called head.sh which reads three lines of text from standard input and outputs those lines to standard output.

For example, called like this:

$ sh head.sh < rhyme.txt
its output would be:

Mary had a little lamb
Its fleece was white as snow.
And everywhere that Mary went
Tip
You may not use any external commands.

You may assume that standard input contains lines of text (it does not contain binary data).

2.3. Pipes
2.3.1. Task — The grep command

In its most basic form, the grep command:

copies its standard input to its standard output,

but quietly drops lines which do not match a pattern.

Here’s an example:

$ grep false < /etc/passwd
...
Here, grep quietly drops lines which do not contain the text false.

Try it for yourself.

2.3.2. Task — The wc command

In the form wc -l, the wc command outputs to its standard output a count of the number of lines found on its standard input.

Here’s an example:

$ wc -l < rhyme.txt
4
Try it for yourself.

2.3.3. Task — Count Password lines

Using the grep and wc commands and a pipe:

count the number of lines in the password file (/etc/passwd) containing the text false.

2.3.4. Task — Count HTML Files

The command:

$ find /usr/share/doc -type f -name '*.html'
outputs a list of all HTML files in or under /usr/share/doc.

How many HTML files are there in or under /usr/share/doc?

2.4. Composition
2.4.1. The test command

The test command tests a variety of conditions, yielding an exit status of 0 if the condition holds, or a non-zero exit status otherwise.

There’s more information here, and in the relevant manual page (man test).

To get the hang of what’s going on, try the following:

$ test -f /etc/passwd
$ test -d /etc/passwd
$ test -s /etc/passwd
$ test ! -s /etc/passwd
$ test -d /var
Tip
If you include the exit status of the most recent command in your shell prompt, then it’s easier to see what’s going on.
2.4.2. Task — Report whether a file exists

For this task, you’ll need to know about the positional parameters.

Write a script file_exists.sh which outputs a message indicating whether a file (provided as an argument) exists or not.

Here’s an example:

$ sh file_exists.sh /etc/passwd
/etc/passwd: does indeed exists
$
$ sh file_exists.sh /etc/does-not-exist
/etc/does-not-exist: does not exist
Note
Use the && and || composition operators; do not use an if statement.
2.4.3. Task — Run command in another directory

Write a script indir.sh which accepts arguments as follows:

$ sh indir.sh DIRECTORY COMMAND [ARGUMENTS..]
Your script should cd to the indicated DIRECTORY and, if that succeeds, run COMMAND with its ARGUMENTS.

Tip
You may need to read about positional parameters.

Your solution should not involve and if statement.

A good solution will also use the exec shell built in command; see the shell manual page (man sh).

3. Advanced Tasks

The following tasks are more difficult. Some require an understanding of material which has not been covered in the course.

3.1. I/O Redirection
3.1.1. Task — script.sh, again

Reconsider script.sh. Extend your script such that, if it is called as follows:

$ sh script.sh FILE -d COMMAND [ARGUMENTS...]
then it inserts the current date and time (see man date) in the output file before running the command.

For example:

$ sh script.sh FILE -d ls /etc
might, in the output file, produce:

Mon 10 Nov 06:49:50 GMT 2014
a2ps.cfg
a2ps-site.cfg
acpi
adduser.conf
adjtime
...
Note
You may use at most one I/O redirection operator.

3.1.2. Task — Standard Output and Standard Error

Complete this task.

Now, call your script in such a way that:

the message intended for standard output appears on standard error, and

the message intended for standard error appears on standard output.

Here’s an example:

$ sh outerr.sh _ADD_YOUR_MAGIC_HERE_ > /dev/null
I am standard output
$
$ sh outerr.sh _ADD_YOUR_MAGIC_HERE_ 2> /dev/null
I am standard error
Where ADD_YOUR_MAGIC_HERE is the same in both cases.

3.2. The read command
3.2.1. Task — tee.sh

Write a script tee.sh with the basic behaviour of the tee utility:

it copies its standard input to its standard output, additionally leaving a copy of its standard input in a file