Tools

Question 1

Libraries:

Static Linking

vs

Dynamic Linking

Answer 1

• Static Linking
  
  • Every routine needed from the library is included in the executable file
  • Routines are PART OF the program
• Dynamic Linking
  
  • References to library routines are left to resolve during execution
  • Library objects not loaded until they are needed at runtime

Question 2

Libaries:

What are

“Shared Libraries”?

Answer 2

Libraries that are prepared for

Dynamic Linking

These are also called

Dynamic Linking Libraries (DLLs)

Question 3

Shell Scripts:

Overview

Answer 3

• A shell script is just a sequence of shell commands
• Stored in an executable text file
  
  • The “x” bit in the file permissions must be turned on with “chmod”
• The shell finds programs by looking for executable files in the path ( $PATH )

Question 4

Shell Utility Toolkit:

Basic

Line Oriented Processing Tools (8)

Answer 4

• grep
  
  • find patterns
• sed
  
  • edit lines
• cut
  
  • extract columns
• sort
  
  • sort lines into specified order
• uniq
  
  • display unique lines in a sorted file
• tail
  
  • display last lines of a file
• head
  
  • display fist lines of a file
• wc
  
  • count words, lines or characters

Question 5

Shell Utility Toolkit:

grep

Answer 5

• Grep is used to find lines or patterns in text
• Use:
  
  • grep [-e pattern]
• This will output any lines that contain any of the patterns

Question 6

Shell Utility Toolkit:

sed

Answer 6

• Used to edit/rewrite lines within the text
• Use:
  
  • sed [-e action]
• Copies input to output, performing all actions on each line
• Possible Actions
  
  • Delete: /pattern/d
    
    • Delete line if pattern found
  • Read: /pattern/r
    
    • Read file after the line where pattern occurs
  • Replace: s/pattern/replace/g
    
    • Substitute “replace” at occurrences of “pattern”

Question 7

Shell Utility Toolkit:

cut

Answer 7

Extract the columns of text

Question 8

Shell Utility Toolkit:

sort

Answer 8

Sort lines of text into a specified order

Question 9

Shell Utility Toolkit:

uniq

Answer 9

Display unique lines in a sorted file

Question 10

Shell Utility Toolkit:

tail

Answer 10

Display the last lines of a file

Question 11

Shell Utility Toolkit:

head

Answer 11

Display the first lines of a file

Question 12

Shell Utility Toolkit:

wc

Answer 12

Counts words, lines or characters in text

Question 13

Shell Utility Toolkit:

File System

Manipulation Tools (8)

Answer 13

• cp - copy
• mv - move
• rm - remove
• ls - list
• chmod - change file access modes
• chown - change file owner
• find - navigate directory structure, testing files for attibutes
• touch - change last-modified time on a file to current

Question 14

Shell Utility Toolkit:

Miscellaneous Tools (5)

Answer 14

• echo - echo arguments on stdout
• cat - copy a file, or files, to stdout
• tr - translate characters
• date - echo current date and time
• man - see manual page of a command

Question 15

Shell Utility Toolkit:

Scripting Languages

Answer 15

In addition to simple shell scripts:

• awk
  
  • Process files based on
    
    • rich pattern matching
    • text manipulation
• perl
  
  • Works similarly, but with more c-like facilities

Question 16

Difference between

C Programming

and

Shell Programming

Answer 16

C Programming deals with:

• Data Structures
• Algorithms

Shell Programming deals with:

• The file system
• Utilities

A professional uses both language types for their unique strengths.

Question 17

Shell Utility Toolkit:

echo

Answer 17

Writes text to stdout

Can pass plain or quoted text

Special Characters:

• *
  
  • represents the contents of the directory
• $
  
  • used for substitution variables
• ’ ‘
  
  • surrounding text with single quotes means special characters are ignored

Question 18

The Shell:

Using pipelines

Answer 18

Use the “|” to connect Filter programs int a pipeline

Example:

% ls -a | sort | uniq

Each step along the pipeline will perform the function

on the results of the previous step

Question 19

Shell Command Format

Answer 19

[command] [-switch] …. [argument(s)] …

• Command file names are located in /bin/
• Built in verbs like “cd”
• Switches are a single letter, such as -a
• can combine switches by adding the letters: -a1
• some switches require you to add a value

Question 20

What doe the Shell Provide?

Answer 20

• Interactive Interface
  
  • Allows use of all system capabilities
  • Navigate and manipulate file system
  • Invoke System Commands and Programs
  • Customize your environment
• Programming Environment
  
  • For system programmers
  • Automate interactions
  • “glue” together smaller programs
  • Prototype solutions

Question 21

The Shell:

Redirection of

Input

Answer 21

By Default, input is normally stdin

Use “<” to specify a different file descriptor.

This allows a file to be used as input.

Example:

% grep -e “abc” < myFile.txt

Question 22

The Shell:

Redirection of

Output

Answer 22

By default, output of programs is to stdout.

Use “>” to redirect the output to a different file descriptor.

This allows output of a program to be written to file.

Alternatively, use “>>” to append the file instead of writing over it.

Example:

% grep -e “abc” > my_output_file.txt

% grep -e “abc” >> my_output_file.txt

Question 23

The Shell:

Redirection of

Error

Answer 23

By default, errors are printed to stderr

Use “ 2> “ to redirect to a different file descriptor

Example:

% grep -e “abc” 2> my_error_file.txt

Question 24

Libraries:

How Dynamically Linked Library

routines are used when called

Answer 24

• The Executable File is by itself
• A call to a routine in the library goes to a Loader routine that has the name of the routine as an argument
• The Loader copies the needed routine from the library and loads it into memory
  
  • Copying may be skipped if the routine is already loaded in memory
• The call is redirected to this routine

Question 25

Dynamic Linking:

Advantages

Answer 25

Advantages

• Size of executable is reduced
  
  • Library routine not present in executable
• Overal system speed increased
  
  • Library routine is only loaded once, then shared
• Program Robustness increased
  
  • Changes to the library don’t require relinking

Question 26

Dynamic Linking:

Disadvantages

Answer 26

Disadvantages:

• Libary version control
  
  • Can the executable find the right version of the library routine?
• “Thrashing” of library routines in memory due to a constrained environment

Question 27

Libraries:

Benefits of Using Libraries

Answer 27

• Save Development Time
  
  • Reusing routines that have already been throroughly debugged
• Save Maintenance Time
  
  • Evolve a coherent capability
  • Using well known libraries with existing best practices

Question 28

Libraries:

Characteristics of

Good Libraries

Answer 28

A Good Library is:

• Complete
  
  • All elements needed for some specific problem are there
• Concise
  
  • Less is more, it is efficient
• Coherent
  
  • Pieces fit together well without undue mental stress
• Correct
  
  • Pieces work in all combinations and usages
• Configurable
  
  • Users can tune and axtend as needed
• Changeable
  
  • Maintainers can evolve the library as the environment changes

Question 29

Shell:

Three Normally Open File Descriptors

Answer 29

• 0: Standard Input, stdin
• 1: Standard Output, stdout
• 2: Standard Error, stderr

Question 30

Parts of the

Executable

Format

Answer 30

• Cookie
  
  • Number that identifies the file as a binary executable file
• Header
  
  • Size of the file in bytes
• Code
  
  • Combined Code Sections
  • Always begins with standard C startup routine from system library
• Data
  
  • Combined Data Sections
  • Contains only initialized external
  • and static internal data
  • Room for uninitialized data set aside during loading
  • includes numbers and strings

Question 31

What does the

Linker

do?

Answer 31

Combines separate

Object files

into a single

Executable File