Problems 3

Shell and Shell Script Skills

You are in your home directory. Give the shortest shell pattern that would match all the files named "UNTITLED" in all the home directories on your machine. (How many of these files are there currently?) You may assume that all home directories are sub-directories under the same directory (though that may not always be true for all Unix systems).

What is the contents of file "hoopla" after the following sequence of commands?
```
    echo first >hoopla
    var=">hoopla"
    echo second $var      
```

What appears on your screen after the following sequence of commands?
```
    var="hello ; echo goodbye"
    echo $var      
```

What appears on your screen after the following sequence of commands?
```
    var=".*"
    echo First "$var" and then $var      
```

Give your Unix command lines to perform the following functions. Where file processing is required, use Unix commands, do not simply read the file into an editor to answer the questions. Your answers must be made up of a Unix pipeline of commands that could read from a file, or from standard input, and write to standard output. For example, to perform the sort operation on a file that has leading numbers on each line, the command would be "sort -n", and it could be used any of these ways:

$ sort -n filename          # sort uses the filename
$ sort -n <filename         # sort reads from stdin
$ cat filename | sort -n    # sort also reads from stdin

Your answers must be able to use standad input in a similar way.

Display the first 5 lines of a file, numbered with line numbers.

Display the lines of a file in reverse order, last to first.
(Hint: sort can sort numbers backward as well as forward, using the right command line option.)

Display everything after the first 5 lines of a file.
That is, start the display of the file on line 6 and go to the end.
(Hint: "man tail")

Display only lines 10-15 in a file. (Try doing this both with and without using sed.)
What happens if the file only has 12 lines? What happens if the file only has 3 lines?

Display from the 20th line from the end of a file to the 15th line from the end of a file.
For example, if the file had 27 lines, your command pipeline would show lines 7 (27-20=7) to 12 (27-15=12). If the file had 127 lines, your command pipeline would show lines 107 to 112. What happens if the file only has 12 lines? What happens if the file only has 3 lines?

The "-s" option to ls gives file sizes (in disk blocks, usually either 512 or 1024 bytes).
Write an executable shell script named "biggest" that shows the five biggest files in the current directory. Make sure your shell script starts with this as the very first line: #!/bin/sh
Put the shell script into your "bin" directory. Hand in a copy of your script.

Look up how to use the Unix "crontab" command. You'll find it in the back of the textbook as well as in the manual system online. Learn the format of the crontab file from the book, or (if you are brave) from the actual man page under "crontab Entry Format".

Create a file containing a crontab entry that executes the following command pipeline (or one similar to it) at noon each day (use your own userid as the write destination):

echo "Time for Lunch" | write abcd0001

You must modify the above line to include the five time fields, as given in the crontab entry format, before you load the file into the cron system.

When you have added the five time fields to the beginning of the above line, use the correct options to the crontab command to load your file into the cron system. Use "crontab -l" to make sure it loaded correctly.

You might want to test that your command works by using different start times for the command, first. (For example, set a start time about 1 minute from now and wait to see if the message appears.)

You can add other crontab commands at other times, if you like. Experiment! Unix system processes use the cron system to run tasks at specific times and on specific days.

If you want to edit your crontab file directly using the "crontab -e" command, rather than loading it from a file, you will be using the Desktop Text Editor unless you set up some other editor using the $EDITOR or $VISUAL environment variables (see text, p. 421/422).

Look up how to use the basic features of the Unix "find" command. You'll find it in the back of the textbook as well as in the manual system online. Look at the Examples; they may be easier to understand at first than the explanation text.

Create and run the following shell script.
```
    #!/bin/sh
    # -Ian!  idallen@freenet.carleton.ca
    find $HOME -mtime -1 -print               
```
Describe what this script is finding. Replace the last argument "-print" with "-ls" and re-run the script, to see more detail on the files and directories found by this command. (You may need to pipe the output of this script into "more" to paginate it on your terminal.) Change the number from "-1" to "1". Re-run the script. Change the number to "+1". Re-run the script. What is the difference between "-1", "1", and "+1"?

Describe what the following Unix command line would do. Note the use of back-quotes for command substitution around the find command and its arguments:
```
    rm `find $HOME/.dt/Trash -mtime +14 -print`               
```
If the find command found a file name with a blank in it, would the above command do the right thing? What if the find command returned a directory name - would the above command function correctly?