Exercises
Exercise 1.1
Suppose ls shows you this:
Makefile biography.txt data enrolment.txt programs thesis
What argument(s) will make it print the names in reverse, like this:
thesis programs enrolment.txt data biography.txt Makefile ls -r
Exercise 1.2
What does the command cd ~ do? What about cd ~hpotter?
"cd ~" changes your working directory to your home directory. "~hpotter" changes the working directory to harry potter's working directory
Exercise 1.3
What command will show you the first 10 lines of a file? The first 25? The last 12? "head -10" "head -25" "tail -12"
Exercise 1.4
What do the commands pushd, popd, and dirs do? Where do their names come from? Reorders the list of directories.
Without an argument such as "+N", "-N", "-n", or "dir", the command "pushd" exchangs the top two directories. popd removes entries from the directory stack. With no arguments,removes the top directory from the stack, and cd's to the new
top directory. dirs displays the list of currently remembered directories. Directories find their way onto the list with the `pushd' command; you can get back up through the list with the `popd' command.
Exercise 1.5
How would you send the file earth.txt to the default printer? How would you check it made it (other than wandering over to the printer and standing there)? "lpr". "lpq".
Exercise 1.6
The instructor wants you to use a hitherto unknown command for manipulating files. How would you get help on this command? use man or bash help
Exercise 1.7
diff finds and displays the differences between two text files. For example, suppose you had a file named earth.txt, that contained the following lines:
Name: Earth Period: 365.26 days Inclination: 0.00 Eccentricity: 0.02
and you modified it so it now looked like this:
Name: Earth Period: 365.26 days Inclination: 0.00 degrees Eccentricity: 0.02 Satellites: 1
You can then compare the two files like this:
$ diff earth.txt earth2.txt 3c3 < Inclination: 0.00 ---
Inclination: 0.00 degrees
4a5
Satellites: 1
(The rather cryptic header "3c3" means that line 3 of the first file must be changed to get line 3 of the second; "4a5" means that a line is being added after line 4 of the original file.)
What flag(s) should you give diff to tell it to ignore changes that just insert or delete blank lines? What if you want to ignore changes in case (i.e., treat lowercase and uppercase letters as the same)?
-B, -i
Exercise 1.8
-rwxr-xr-x 1 aturing cambridge 69 Jul 12 09:17 mars.txt -rwxr-xr-x 1 ghopper usnavy 71 Jul 12 09:15 venus.txt
According to the listing of the data directory above, who can read the file earth.txt? Who can write it (i.e., change its contents or delete it)? When was earth.txt last changed? What command would you run to allow everyone to edit or delete the file?
aturing, cambridge, and all others can read. aturing can edit. aturing, cambridge, and all others can execute it. it was last changed on July 12 at 9:17 AM. chmod g+w. chmod o+w.
Exercise 1.9
Suppose you want to remove all files whose names (not including their extensions) are of length 3, start with the letter a, and have .txt as extension. What command would you use? For example, if the directory contains three files a.txt, abc.txt, and abcd.txt, the command should remove abc.txt , but not the other two files.
rm a??.txt
Exercise 1.10
You're worried your data files can be read by your nemesis, Dr. Evil. How would you check whether or not he can, and if necessary change permissions so only you can read or write the files?
chmod o-rwx
Exercise 1.11
What's the difference between the commands cd HOME and cd $HOME?
"ch $ home" will take you to your home directory
Exercise 1.12
Suppose you want to list the names of all the text files in the data directory that contain the word "carpentry". What command or commands could you use?
grep carpentry
Exercise 1.13
Suppose you have written a program called analyze. What command or commands could you use to display the first ten lines of its output? What would you use to display lines 50-100? To send lines 50-100 to a file called tmp.txt?
analyze myfile | head -100 | tail -50 > tmp.txt
Exercise 1.14
The command ls data > tmp.txt writes a listing of the data directory's contents into tmp.txt. Anything that was in the file before the command was run is overwritten. What command could you use to append the listing to tmp.txt instead? use two "greater than" (>) signs
Exercise 1.15
What command(s) would you use to find out how many subdirectories there are in the lectures directory? find ./lecture -type 'd'
Exercise 1.16
What does rm *.ch? What about rm *.[ch]? remove all files with the last three characters ".ch" removes all files ending in .h and all files ending in .c
Exercise 1.17
What command(s) could you use to find out how many instances of a program are running on your computer at once? For example, if you are on Windows, what would you do to find out how many instances of svchost.exe are running? On Unix, what would you do to find out how many instances of bash are running?
ps aux | grep ac07bac ps aux |grep bash
Exercise 1.18
A colleague asks for your data files. How would you archive them to send as one file? How could you compress them?
tar -cf data.tar Data gzip data.tar
Exercise 1.19
You have changed a text file on your home PC, and mailed it to the university terminal. What steps can you take to see what changes you may have made, compared with a master copy in your home directory? diff version 1 against version 2
Exercise 1.20
How would you change your password?
passwd
Exercise 1.21
grep is one of the more useful tools in the toolbox. It finds lines in files that match a pattern and prints them out. For example, assume the files earth.txt and venus.txt contain lines like this:
Name: Earth Period: 365.26 days Inclination: 0.00 Eccentricity: 0.02
grep can extract lines containing the text "Period" from all the files:
$ grep Period *.txt earth.txt:Period: 365.26 days venus.txt:Period: 221.70 days
Search strings can use regular expressions, which will be discussed in a later lecture. grep takes many options as well; for example, grep -c /bin/bash /etc/passwd reports how many lines in /etc/passwd (the Unix password file) that contain the string "/bin/bash", which in turn tells me how many users are using bash as their shell.
Suppose all you wanted was a list of the files that contained lines matching a pattern, rather than the matches themselves—what flag or flags would you give to grep? What if you wanted the line numbers of matching lines?
-x -n
Exercise 1.22
Suppose you wanted ls to sort its output by filename extension, i.e., to list all .cmd files before all .exe files, and all .exe's before all .txt files. What command or commands would you use?
'
Exercise 1.23
What does the alias command do? When would you use it?
alias lets you make one command look like another. You would use it to shorten commands so they are more convenient to use.
