Class 5: The Bash shell - a refresher

Held: Thursday, 23 February 2017

We explore some key aspects of our primary shell

Preliminaries

Overview

• 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

Related Pages

• Eboard 5 (Source)

Updates

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 Linux
• Friday Jazz band
• Friday-Saturday diving - Cheer on future-president White
• Saturday morning film
• Sunday swimming
• All things in moderation

Some Simple Shell Stuff

• To use *nix, you need to be proficient in the basics of shell usage.
  • I’ll focus on bash, the Bourne-again shell
• What do I consider “the basics”?
• You should know how to make and use files and directories.
• You should know about file permissions (and how to set them numerically or mnemonically)
• You should understand basic ways to deal with redirecting program I/O
  • You should know how to redirect the output of a program.
    • To a file
    • To another program
  • You should know how to redirect the input of a program.
    • From a file
    • From another program
  • You should know about the differences between stdout and stderr and how to take advantage of those differences.
• You should know about command-line patterns
• You should know how to set and get normal shell and environment variables (and why you how might do so).
• You should know how to take advantage of the shell history.

Files and Directories

• ls
  • And its flags.
• mkdir
• rmdir
• rm
  • Exercise: Remove the file named -f, remove the file named *.
• cd vs pushd
• Links, soft and hard
  • Activity: We’ll explore effects of deleting files with the different kinds of links.
• du

Command-Line Patterns

• * - everything
• *.jpg - all the files that end with .jpg
• {alpha,beta}.jpg - alpha.jpg and beta.jpg
• e?.jpg - e1.jpg and e2.jpg but not echo.jpq

File and Directory Permissions

• Three parts: User, group, other
• Each part has three settings: Read (4), Write (2), Execute (1)
• Question: What do the parts mean?
• Question: What do the parts mean for directories?
• Special thing: The setuid bit. What do you think it does?

I/O Redirection

• Why?
• Files vs. Pipes
• Stdout vs. Stderr
  • How do you combine them.

Variables

• Two kinds: Standard environment variables and user-defined variables.
• Used like all variables: To give a name to information that may vary (e.g., from user to user).
• Generally set with a command like
  *var*=*value*
  • No spaces are allowed in the value, so when you need spaces, put the value in quotation marks.
• Access variables as $*var*
• Environment variables are shared from program to program.
• May need to export enviornment variables.
  export *VAR*="*value*"
  or
  export *VAR*
• List all current environment variables with printenv

Command-Line History

• Bash (and the *nix shells) keeps a history of the commands you’ve typed.
• You can repeat a command with an exclamation point and command number, such as !10.
• You can repeat the previous command with !!.
• You can repeat a command that begins with a particular string with !*str*. I use !make a lot.
• You can extract the last “word” on the previous line with !$.
  • I might write *program* > *file* and then less !$, particularly if the file has a long pathname.
  • I might use cp *file* *dir* followed by a cd !$.
• Command-line history is also useful with the command line browser (up and down arrows, plus editing).

The script Command

• Sometimes you want a record of what you’ve done.
• The script command starts a new shell and records input, output, and more.
• Finish with -D.
• But leaves “garbage” in the file (see example).

Bash Control

• Bash is a programming language. At times, you’ll want to write scripts to do useful tasks.
• Examples
  • Rename all the files that end with .jpeg to .jpg