6 January 2022

6 January 2022

`vi` Review

Modes:
    visual:  line, block, chararacter (texticographical)
        selects text
    insert
        type in new text with the kb
    command
        search
        search/replace
        copy/paste
        mobility
        save
        quit
        read in a file
    replace
        mows down text
        r then a character puts tht character at the cursor.
ESC key:  command mode

Permissions

You own your home directory

Who owns what?

How do you allow/restrict use of files by others?

-rw-rw-r-- 1 fake23z fake23z 0 Jan  6 09:12 somefile

- at beginning: regular file
d at beginning: directory

next three:  your permissions (u)
    read   (r)
    write  (w)
    execute(x)
next three: your group's permissions
    read 
    write 
    execute
next three : others (on this computer)
    read
    write (ugh)
    execute

chmod (a|u|g|o) (+|-) filename

Octal representation for permissoins
Three digits, first is u, second is g, third is o.

r 4
w 2
x 1
none: 0

Directories:
    Execute permission must be on or the directory is opaque.
    this blocks access to all contents of the directory with no execute permisison

Writing a Shel Script

Path Exercise

Make a directory called zoo and cd into it.

Path Exercise Proboem

You will write two shell scripts. In one, you construct the tree shown but you never cd into another directory. Use paths.

In the second, you construct the tree shown but you will cd into each directory to make its children. Make sure you end up back in zoo.

Can you do the whole thing using only absolute paths and by never leaving your home directory?

Making a Web Page

Yep, you will learn some very basic HTML and learn about permissions needed for a web page to work. Once yours is working, the link will be visible here.

Apache is a user on this computer, just like you
Apache is the web server
    Apache gets a request for a page
    Apache fetches that page (and needs permission to do so)
    Apache sends the page out onto the network.

common configuration
    public_html is the www directory
    This is the www subtree for your accont
    Any subdirectory can have an index.html file which shows by default

Redirection

> redirects output into a file.  Any existing file clobbered
     any new file is created automatically
>> appends output to a file.
< redirects standard input. 

cat < file  makes file cat's stdin.  same as cat file

2> redirects stderr to a file

All Unix filters have
    stdin: standard input (default: kb)  file descriptor:  0
    stdout: standard output (default: screen)  file descriptor: 1
    stderr: standard arror (default: screen)   file descriptor:

UNIX filters work line-by-line


cat:  the non-filter filter that puts the entire file to the stdout
      -n: put line numbers

sort:  sorts things alphabetically
        -n to sort numericallly
        -g to sort with floats
        -r to sort in reverse

head:  show top of a file, by default 10 lines
        -(number) shows specified number of line

tail:  show bottom of a file, by default 10 lines
        -(number) shows specified number of line

grep:  grep string file
       all lines from the file conaining the specified string
        are put to stdout.

uniq:  This eliminates neighboring duplicates.        

wc:    counts words, lines and characters
wc -l:  number of lines
wc -c:  number of characters
wc -w:  number of words

PUZZLER:
find the word on line 50000 in the system dictionary!



PUZZLER:
put the phrase "this is written in magic ink
into a file without using a text editor



Regular Expressoins

These specify textual patterns.



Clue:  butter

Xg..tX   crossword cheating!

word containing a, e, i, o, u and y in that order

How do we specify these?

Character class: This is a wildcard for characters

[abcqm]:   matches any of a, b, c, q, or m  (list character class)
[a-z]      matches any lower case letter (asciicographical range)
[^a-z]     sNOTTY hat at start means "anything but"
[a-zA-Z]   two ranges
[^a-zA-Z]  snotty with two ranges.  

\d  matches a digit
\s  matches whitespace
.   matches anything but a newline
\S  matches any non-whitespace
\D  matches any non-digit

^  matches start of line
$  matches end of line
\b matches a word boundary
Magic:
\ toggles magic

Juxtapositition:  "and then immediately"

Multiplicity (postfix unary operators):
    {n}     matches "n times"
    {m, n}     matches at least m times but not more than n
    {m,}    at least m of
    ?       once or not at all
    +       one or more
    *       zero or more
delimiters:
    ( and ) override the order of operations
Phone number:  (XXX)XXX-XXXX

\([2-9][0-9]{2}\)[2-9][0-9]{2}\-[0-9]{4}
\([2-9]\d{2}\)[2-9]\d{2}-\d{4}

check if a string is a legit integer

starts                     ^
possibly a sign            [+\-]?
digits: 1 or more          \d+
ends                       $

^[+\-]\d+$                 

PUZZLER:
find all words in system dictionary with a, e, i, o u, and y in order
appearing in the word.



last class
next class
 Back to Calendar View