UNIX Notes

UNIX Tools and Tricks

Editing
- There are 3 ways for interactive editing text under UNIX:
  - The vi way
  - The emacs way
  - The wrong way
- vi (the visual editor) is the text editor written by Bill Joy for BSD UNIX (published about 1978)
  - Screen-oriented WYSIWYG editor (for plain text)
  - Available on just about any UNIX system
  - About 35% of all serious UNIX hackers still prefer vi (or a derivative)
  - vim is an improved version
    vimtutor will get you going
  - gvim is a windows based interface to vim
- emacs (editing macros) started in 1976 as a set of TECO macros on ITS
  - Currently popular emacs versions (GNU Emacs and XEmacs) go back to 1985 GNU Emacs by Stallman. Both basically are a LISP system with a large text editing library and an editor-like user interface
  - About 35% of all serious UNIX hackers use Emacs. Also widespread use on other operating systems
  - emacs has zillions of extensions
    ^H t wil get you going
- pico is a wrong way
  nano is a wrong way
  gedit is a wrong way
  kedit is a wrong way
  Anything other than vi or emacs is a wrong way!
Less typing
- History
  - Set the number of commands to store in your shell resource file, e.g., set history=100.
  - history lists recent commands. Gurus alias that to "h".
  - !<number> redoes that command
  - !<partial> redoes the most recent command that started with <partial>
- Globbing
  - Describe to the shell (not OS) sets of file names with patterns
  - ~ means the home directory
  - ? means any character
    * means any string (including empty)
  - [<letters>] means any one of the <letters> (except ...), which can also be a range
  - ^ negates some glob expressions
- File name completion
  - Many shells, including bash, provide file name completion
  - Use tab to expand partial file names
Foreground and background processes
- Ooooh, the Ticker
- Foreground processes are started by typing a command. While a foreground process is running the shell waits.
- Background processes are started by appending a & to the command.
  - While a background process is running the shell continues.
  - Output from the background process is mixed in ... the solution is to send the stdout to a file, leaving stderr to screen.
  - If a background process needs input, it gets suspended (read on).
- Each process you start from the shell is known as a "job", i.e., you have maximally one foreground job and can have multiple background jobs.
- Background processes can be controlled
  - jobs lists background processes, and their numbers
  - kill %N terminates a background process by job number
  - fg %N brings a background process into the foreground
- The foreground process can be suspended and moved to the background
  - ^Z suspends the foreground process
  - jobs also lists suspended processes
  - bg %N activates a suspended process in the background
  - fg %N also activates a suspended process in the foreground
  - kill %N also terminates a suspended process
- A background process will be suspended automatically if it needs to read data from stdin.
- nohup can be used to leave jobs running when you logout ("hangup").
  - disown is a lesser version, with different effect
Sequences and groups
- Simple commands are commands
- Pipelines of simple commands are commands
- Sequences of simple commands with & are commands, and will be executed concurrently.
- Sequences of simple commands with ; are commands, and will be executed sequentially.
- Commands in ()s are simple commands.
Manipulating files
- find searches for files
  - -name to find a file by name
  - -empty to find empty files and directories
  - -type to find files of various types (bcdflps)
- more and less display a file one page at a time
  head and tail display the start and end of a file
  od can display non-text files
- diff compares text files
  wc counts characters, words, lines
Regular expressions
- Regular expressions describe sets of strings with patterns (not the same as globbing)
  - A normal character matches itself
  - . matches any normal character
  - A range [<letters>] matches any one of the <letters> (except ...), which can also be a range
    [^<letters>] matches any not one of the <letters>
  - ? after a pattern makes it optional
    + after a pattern matches one or more repetitions
    * after a pattern matches any number of repetitions
    {<N>} after a pattern matches <N> repetitions in regexps
  - ^ means the start of the line
    $ means the end of the line
  - ()s round a regexp makes it one thing to which repetition and placement options can be applied.
- regex101 is a great tool to play and learn regular expressions.
Finding file contents with grep
- grep finds lines in files that match limited regexps.
- grep -E finds lines in files that match a regexp
  - grep -E "^[a-zA-Z]+" looks for lines that start with some alphabetic characters.
  - grep -E "a*b+c{4}" looks for lines containing any number of as followed by some bs followed by at least 4 cs
  - grep -E '^(a*b+c{4})+$' looks for lines containing exactly repetitions of the abcs (note the use of ''s to avoid confusion with shell variables)
- Example: Extract the legal, Java style, int and double variable declarations from GrepExVars.
  grep -E '^\s*(int|double)\s+[a-zA-Z_]+(\s*,\s*[a-zA-Z_]+)*\s*;' GrepExVars
- Example: Extract the University of California lines from GrepExUC.
- grep has some useful options
  - -c to count number of matches
  - -l to list files names that match
  - -v to list lines that don't match
Exercises
- Set your history to store the last 100 commands, and install an alias h to list your history.
- Is globbing a cool word or what?
- Find out how to use name completion in your shell.
- Copy and compile the Ticker.c program. Then do the following sequence
  1. Start a ticker with a 7 second delay, in the foreground
  2. Suspend the ticker
  3. List your jobs
  4. Start a ticker with a 9 second delay in the background
  5. List your jobs
  6. Activate the 7 second ticker in the background
  7. List your jobs
  8. kill the 9 second ticker
  9. kill the 7 second ticker
- Use find to make a list of all the empty files in your account.
- Provide a grep command to extract the legal (both syntactically and with correct identifier style) integer variable definitions from the following lines:
```
    int aDog;
int   aDog  ;
//  int aCommentAboutADog;
    double aBigDog;
    int BadDog;
    int dogWithNoTail
    int aDog,aCat;
    int   aSpaceDog,   aSpaceCat;
    int aDog aBadCat;
    internationalDog;
    int a#Dog;
int internetName; // fooo
```
Exam Style Questions
- At a command prompt, what would the following commands do:
```
!17
!gli 
```
- In globbing, what do the * and ? characters designate?
- Explain the difference between a background process and the foreground process.
- Explain the difference between these two commands
```
ls & ps & date &
ls ; ps ; date 
```
- What does the following command output?
```
find / -empty | wc -w 
```
- Which of the following lines are recognised by the regexp ^a*b+c{2}?
```
bbbbcc
abbccc
  aaabbbccc
aaabbbcccddd
bccbccbccbccbcc 
```

UNIX Tools and Tricks

Exercises

Exam Style Questions