Chapter 7: Advanced Shell Programming

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Chapter 7Advanced Shell Programming
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Ensuring the Correct Shell Runs the Script

• Each UNIX/Linux user can choose which shell they
  use by default
• Ensure the correct shell is used to run a script
• Not all shells support the same commands and
  programming statements
• The first line of a script should specify which
  shell to use

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Setting the Default Shell

• System administrator establishes the default
  shell for a user account
• User shell set in /etc/passwd file
• File can only be edited (carefully!) by system
  administrator
• Some systems provide management software to
  assist in setting default shells

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Using Bash Login and Logout Scripts

• When Bash is your default shell, scripts run
  automatically upon login or re-entry
• .bash_profile
• .bashrc (also runs in a subshell)
• Administrator controls /etc/bashrc and
  /etc/profile
• .bash_logout runs when user logs out
• Often used to clear the screen

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Defining the Shell (Sha-Bang)

• A shell script is little more than a list of
  commands that are run in sequence.
• Conventionally, a shellscript should start with
  a line such as the following
• !/bin/bash
• This indicates that the script should be run in
  the bash shell regardless of which interactive
  shell the user has chosen.
• This is important, since the syntax of different
  shells can vary greatly.

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A Simple Example

• Here's a very simple example of a shell script
  with comments. It just runs a few simple commands
  
• !/bin/bash
• Tell the user what is happening
• echo "hello, USER. Here is a directory listing
• list files
• echo "listing files in the current directory,
  PWD"
• ls

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More on Shell Variables

• !/bin/bash
• XABC
• echo "Xabc"
• This gives no output. What went wrong ?
• The answer is that the shell thought that we were
  asking for the variable Xabc, which is
  un-initialized.
• The way to deal with this is to put braces around
  X to separate it from the other characters.
• The following gives the desired result
• !/bin/bash
• XABC
• echo "Xabc"

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Setting Defaults for Using the vi Editor

• To use the vi editor for code development,
  configure .exrc in your home directory
• Automatically sets up the vi environment
• Set the number of tab spaces to use when nesting
  lines of code
• Display line numbers

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Some vi tips for .exrc

• To view the configurations of the parameters in
  your current vi session, go into the command mode
  and issue the command set all .
• set all
• or
• setall

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Some vi tips for .exrc

• Setting syntax on/off
• syntax off
• Precede each line with a number
• set number
• Set word wrap 3 characters from edge of screen
• set wm3
• Create an abbreviation/shortcut
• ab wwr William W. Richards

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Spell Checking with vi

• map V wM!ispell Me!MM

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Reading in the Date

• ab DATE M.!date '\a \d \h \Y'MkJA

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Clearing the Screen

• The clear command is useful for clearing the
  screen, but there is a faster way
• Store the output of the clear command in a
  variable and then echo the contents of the
  variable on the screen
• About ten times faster than the actual command
  since the system does not have to locate and
  execute the clear command

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Clearing the Screen

• How do you do this?
• Simple
• CLEAR/usr/bin/clear
• export CLEAR
• echo CLEAR

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Chapter 8Exploring UNIX/Linux Utilities
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Objectives

• Understand many of the UNIX/Linux utilities that
  are available and how they are classified
• Use the dd utility to copy and convert files
• Make a bootable removable disk
• Monitor hard disk usage

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Objectives (continued)

• Use system status utilities
• Monitor and manage processes
• Check the spelling of text in a document
• Use the cmp command to compare the contents of
  two files
• Format text to create and use a man page

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Understanding UNIX/Linux Utilities

• UNIX/Linux utilities let you
• Create and manage files
• Run programs
• Produce reports
• Monitor and maintain the system
• Recover from a range of errors
• New utilities are continually being added in
  order to make UNIX/Linux run more efficiently

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Understanding UNIX/Linux Utilities (continued)

• Classified into eight major areas
• File processing
• System status
• Networking
• Communications
• Security
• Programming
• Source code management
• Miscellaneous

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Classifying UNIX/Linux Utilities
cat Display files fmt Formats text
cp Copy files grep Matches patterns in files
cpio Copy/back files fgrep Matches patterns in files
cut Selects char/fields gzip Zip/compress files
dd Convert/copy a file/image gunzip Unzip/uncompress files
dump Backs up files head Displays 1st part of files
file Displays file type ispell Spell Checks
find Finds files less Displays files
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Classifying UNIX/Linux Utilities (continued)
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Classifying UNIX/Linux Utilities (continued)
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Classifying UNIX/Linux Utilities (continued)
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Classifying UNIX/Linux Utilities (continued)
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Classifying UNIX/Linux Utilities (continued)
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Classifying UNIX/Linux Utilities (continued)
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Classifying UNIX/Linux Utilities (continued)
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Using the dd Command

• Allows you to copy a file and change the format
  of the destination file
• Has a rich set of options to handle copies when
  other methods are inappropriate
• An advantage to using the dd command over cp is
  that all users, not just the administrator, can
  copy files to and from the floppy drive

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Making a Bootable Removable Disk

• Make a bootable floppy disk because a computer
  problem may prevent you from starting UNIX/Linux
  from the system
• Bootable floppy disks or CD-ROMs can be made
  using utilities provided by your version of
  UNIX/Linux
• Such as the mkbootdisk command
• mkbootdisk 2.6.17-1.2145_FC5

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Making a Bootable Removable Disk (continued)

• Find the kernel version so you can create a
  floppy boot disk try these commands
• uname r
• cat /proc/version
• ls l /lib/modules

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Checking Hard Disk Usage

• To maintain adequate hard disk free space, use
  these strategies
• Be vigilant against running dangerously low on
  free space by using the df command
• Watch for conspicuous consumption using the du
  command
• Follow a routine schedule for garbage
  collection and removal by using the find and rm
  commands

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Using the df Utility
The df utility reports on the status of1024-byte
blocks that are allocated, used, and available
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Using the du Utility
The du utility summarizes disk usage, expressed
in 512-byte blocks (default) or by the number of
bytes(-b option)
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Removing Garbage Files

• Garbage files are temporary files that lose their
  usefulness after several days
• Two examples of garbage files are core files
  (named core) and a.out files
• Use the find command to assist you in locating
  these files and the rm command to remove them

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Removing Garbage Files (continued)

• Create some files
• touch core a.out
• Now use the find command to find and delete
• find . ( name a.out o name core ) exec
  rm \
• Note the ( and ) are so the command can be
  used to delete more than one file name

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Using System Status Utilities

• System status commands reflect the systems
  performance
• System engineers primarily use the data related
  to system status
• Good to know how to obtain and store relevant
  information to send to system administrator and
  tune-up specialists

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Using the top Command

• One of the most effective utilities for auditing
  system performance is the top command
• The top command displays a listing of the most
  CPU-intensive tasks in real time
• Updates every five seconds by default

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Using the top Command (continued)
The top utility run without any options specified
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Using the uptime Command

• uptime tells you how long a system has been
  running since the last time it was booted
• Displays current time
• How long the system has been up
• Number of users on the system
• Load average for 1, 5, and 15 minutes

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Using the free Command
The free utility displays the amount of free and
used memory in the system
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Forwarding top and free Output

• When problems arise with performance, may need to
  forward top and free output to support person
• Use redirection (gt) to store outputs in files
• top n 1 gt top.txt
• free gt free.txt

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Managing Processes

• A process is identified through a unique number
  called a process id (pid)
• Unix/Linux offer utilities to run, monitor, and
  kill processes using pids

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Running Processes in the Background

• Can run a process in the background while working
  with another program in the foreground
• To run a program in the background, append the
  character to end of the startup command, e.g.,
  top

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Monitoring Processes
The ps command with the -A option shows a list of
all system processes currently running
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Killing Processes

• Administrator with root privileges can kill any
  users processes
• User can kill owned processes
• Use kill command with the pid of the process
• Use kill 9 to stop a process that doesnt
  respond to an initial kill command
• See man page for descriptions of the signals
  (remember the man trick)

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Checking the Spellingof a Document
ispell scans a document, displays errors on the
screen and suggests alternative spellings
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Comparing Files

• Use the cmp utility to compare the contents of
  two files, and report the first difference
  between them
• The cmp command displays the position and line
  number of this difference
• If there are no differences, the cmp command
  displays nothing

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Formatting Text in UNIX/Linux

• Text formatting in UNIX/Linux involves preparing
  a text file with embedded typesetting commands
  and then processing the file
• UNIXs nroff and troff commands were the early
  standard in formatting programs
• An embedded code is a special sequence of
  characters that is included with the regular text
  of the file

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Formatting Text in UNIX/Linux (continued)
Linux introduced groff, which implements the
features of both nroff and troff
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Formatting Text in UNIX/Linux (continued)
Groff can be used to produce a man page that
contains the standard man page sections
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Formatting Text in UNIX/Linux (continued)
Man pages are made available to others by having
a privileged user copy it to one of the man page
directories
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Chapter Summary

• UNIX/Linux utilities are classified into eight
  major functional areas
• Utility programs are called commands executed by
  entering names on the command line
• dd command options allow it to handle copies when
  other copying methods fail
• To make a bootable removable disk, use provided
  utilities such as mkbootdisk

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Chapter Summary (continued)

• df checks and reports on free disk space
• du checks for disk usage
• Use find to retrieve temporary files and use rm
  to remove them
• top and free provide detailed views of the
  internals of the system that can be redirected
  to a file for system tune-up

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Chapter Summary (continued)

• Run a program in the background by appending to
  the end of a command
• ps displays all running processes
• kill terminates a specific process
• ispell scans for spelling errors
• Text formatting involves
• Embedding typesetting commands in a file
• Processing the file with a program that generates
  commands for the output device

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Chapter Summary (continued)

• Linux introduced groff, which implements the
  features of both nroff and troff
• Text formatted with groff can be used to create
  new man pages