CSC 282.01, Class 05: The Bash shell - a refresher

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Questions
• Things you should know about Bash
• Explorations of those things

News / Etc.

• Sorry for getting homework out late this week. Life continues to be
  more chaotic than I thought it would be.
• We should be doing Make next week.

Upcoming Work

Forthcoming

Good things to do

• Today’s CS Extra on MathLAN
• Friday Jazz band
• Friday-Saturday diving - Cheer on Summer White
• Saturday morning film
• Sunday swimming
• All things in moderation

Things you should know about Linux

• Files and directories
• Links, soft and hard
• File and directory permissions
• I/O redirection
• Command-line patterns
• Shell Variables
• Command-line history
• The script command

Files and directories

• Remember: We have storage.
• Storage is broken into chunks - inodes
• Files go in those chunks
• Directories also have to go into chunks
  • Directories list
    • Name, Address of each file and subdirectory
• inodes must store not just data, but also metadata
  • Maybe file type (can be derived from contents or extension?)
  • Creation time, modification time
  • File size
  • File permissions
• What happens if you have more data in a file than fits in one inode?
  • Option 1: Linked list of inodes
  • Option 2: Tree of inodes
• You don’t need to know all of the details, but it helps to think about and remember some of these issues.

Links, soft and hard

• Yay pointer manipulation
• ln source target - Hard link, both point to the same inode
• ln -s source target - Soft link, points to the directory entry
  • Necessary for directories.
  • Different contexts require different choices
• Example in which soft links are useful
  • vi exam1
  • ln exam1 023123.rkt
  • rsync … exam1 (exam1 gets a new inode)
  • 023132.rkt refers to the old inode
  • ln -s ~/public_html ~/Web
• Example in which hard links are useful
  • Whoops! I didn’t mean to delete that.

Explore the difference between soft links and hard links by creating a file and hard and soft links to that file. Then try each of the following.

• Edit the original file and determine which file contents change.
• Edit the hard-linked alias, and determine which file contents change.
• Edit the soft-linked alias, and determine which file contents change.
• Delete the original file and determine what happens if you attempt to look at the other two files.
• Recreate the original file (same name, different contents) and see what happens to the other two files.
• Remove the two aliases and re-link for the next steps.
• Overwrite the contents of each file, using a command like echo "Goodbye" > FILE and then determine which changes affect which files.
• Summarize what you’ve learned.

If you’ve done that already

• Explore soft links to soft links
• Explore soft links to non-existant files
• Check permissions issues related to these things. What if the original is not readable, but the linked version is?

Conclusion

• If the contents of the inode changes, we see it through the hard link
• If the inode associated with the filename changes, we see it through the soft link, but not the hard link.
• If we rename the file, we can still see it through the hard link, but not the softlink.

Anecdote

• Even experts get things wrong.
• Things may have changed.

File and directory permissions

I/O redirection

Command-line patterns

Shell variables

Command-line history

The `script command

• The [rebelsky@church 501] ~/Web/Courses/CSC282/2017S/_dev/_eboards $ ^C [rebelsky@church 501] ~/Web/Courses/CSC282/2017S/_dev/_eboards $ ^C [rebelsky@church 501] ~/Web/Courses/CSC282/2017S/_dev/_eboards $ ^C [rebelsky@church 501] ~/Web/Courses/CSC282/2017S/_dev/_eboards $ exit Script started, file is typescript Script started, file is typescript Script done, file is typescript command