EBoard 02: Thinking about Unix

This class may be recorded! Its use will be limited to members of the class. Please do not share with others.

Approximate overview

• Very little administrative stuff
• Review ideas of Unix Tools
• Review ideas of Unix Tasks
• Review ideas from TAoUP Chapter 1

Administrative stuff

General Notes

• The class Web site is still at https://rebelsky.cs.grinnell.edu/Courses/CSC282/2022Sp/.
  • The class Web site is (always) a work in progress.
• Sorry about the GitHub confusion. Let me know if you’re still having trouble.

Upcoming Activities

• Thursday extras today! Learn about the 4+1 program.

Work for next Thursday

• Read Sam’s comments on formatting C code
• Skim Making the best use of C
• Read Chapter 1 of Managing Projects with GNU Make
• Read Rob Pike’s Notes on Programming in C
• Think about how you would do unit testing in C.

Fun with GitHub

• Did anyone encounter merge conflicts?

Review Unix Tools

• https://github.com/grinnell-cs/csc282-2022sp/blame/main/unix-tools.md
• I would have organized the file differently, but …
• We’ll read through it a bit; I may also make comments on your Git Commits and other issues.

Git notes

• Don’t do independent changes in one commit.
• Please use sensible commit messages.
• When working in a shared file, PUT STUFF IN THE RIGHT PLACE!
• Do not add cruft to the repo.

Awk is good for text processing

• Example: Sam downloads class list in "NAME","ID","STANDING","EMAIL" form
• Sam usually wants class list in "NAME" <EMAIL> form or “FIRST”,”LAST” form.
• If it’s a task Sam will do regularly, Sam should write an Awk script (or maybe a Sed script, since it’s pretty simple).
• Note: “NAME” to “FIRST LAST” is difficult.

cd changes directories

• Relative or absolute
  • cd /home/rebelsky changes to my home directory. (absolute)
  • cd fizzbuzz changes to the fizzbuzz subdirectory. (relative)
  • cd ../fizzbuzz changes to the fizzbuzz sibling directory. (relative, using the magic of dotdot, which means “parent directory”)
• cd’s only command-line flags seem to have to do with following symbolic links.
• What’s a symbolic link?
• Maybe we should ask what a link is, first.

Links in Unix

• A link is effectively an alias for a file (or other file-like thing, such as a directory).
• That is, it’s another name for the same file (or …)
• If we create the link with ln source dest, it’s a hard link.
  • Modifications to the file through either name affect the other because they are the same file.
  • In essence, the symbol table maps both names to the same area on disk.
• We normally can’t create hard links
  • To directories $ ln foo bar ln: foo: hard link not allowed for directory
  • To files on other disks
  • Both have technical barriers.
• Why link files?
  • We do it a lot: E.g., Make a link to this application on my desktop.
  • Sometimes we have a library we need in lots of folders (perhaps because we’ve made bad design decisions).
  • Links help ensure that if one thing changes, everything changes.
• Unix also allows “soft links”. These links don’t reference a place on disk (or equivalent): They just use the path to reference the place.
• We use ln -s source dest for soft links.
• We can see soft links with ls -F <FILE>.
• We can see soft links with file <FILE>.
• Like paths for cd, symbolic links can be relative or absolute.
  • If you make a symbolic link, and move it, it now links to something different.

cat vs. more (less)

• cat is intended to show the contents of files.
• cat <FILE1> <FILE2> ...
• more is a pager. It shows one screenful of a file at a time.
• We can use cat as a quick way to create files.
  • cat > filename.txt
  • That’s because by default, cat takes standard input as the stuff to type.
• cat is short for “concatenate”
• cat file1 file2 file3 > file4 will combine the contents of the first three files onto the last.

ls <DIR> lists the contents of a directory

• If the directory is not specified, lists the current directory.
• -l gives a long listing, which includes permissions and other info, including owner, group, last modification, sizeish, …
• -a also shows “hidden” files. Convention on Unix systems is that files and directories that begin with a dot are not shown unless you use this flag.
• -F adds a character after certain kinds of files
  • / after directories
  • @ after symbolic links
  • * after executables
• -d does not expand directories
• -H follows symlinks
• -R recursive
• -t in order of modification time
• -G might use color to show differences (like -F)

Permissions will come another day

• There is a (disallowed in MathLAN) “setuid” permission, which makes an executable run as the owner of the file, rather than the person who executed the file.

ps and top list processes in different ways.

• ps gives all of them
• top gives the top ones

Review Approaches to Problems

Given a DOS-formatted text file (lines end with \r\n rather than just \n),

convert it to a standard text file

Given a standard text file, convert all uppercase letters to lowercase

Given a standard text file, remove all blank spaces at the end of lines

Make a list of all misspelled words in a text file

Find the names of the five highest grades on homework 2

Lines have the form

 LastName,FirstName,Assignment,NumericGrade 

Given an HTML file, find the URLs of all images.

Generalize Approaches

Ideas from TAoUP