Introduction to Bash Programming

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Introduction to Bash Programming

• Ellen Zhang

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Previous three classes

• What have we learnt so far ?

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Outline

• Shell command line syntax
• Shell special characters, metacharacters
• Write our own echo command
• Use echo to experiment with metacharacters
• Quotation to protect metacharacters
• Practice problems
• Our first shell script
• Passing arguments to shell script
• Generate arguments by running a command

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Shell command line

• A command ends with a newline, or a semicolon
  (), or an ampersand ()
• date
• date who
• sleep 20who
• Sending two commands through a pipe
• date who wc
• What happens ?
• ls l grep d

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Shell command line

• Use parenthesis to group commands
• (datewho) wc
• sleep command pause for given amount of time,
  and then return
• How to show a message in the console to yourself
  in 30 minutes ? get up and do some exercise !
• (sleep 1800 echo get up and do some exercise!
  )
• We will learn how to put this into a loop

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Useful commands

• tee copies its input to a file and to its
  standard output
• (date who) tee save wc

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Special SHELL characters

• Characters in the command line that shell
  interpret them and substitute them
• gt, gtgt, lt, , , , !
• making the rest of the line comments (must be
  the first letter of a word)
• var means value of variable var
• match any string of zero or more characters in
  file name (except those starting with .)
• ?, aeiou, 1-9, a-z, 0z,a-z

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Remember echo command

• echo display a line of text
• echo Good morning, everyone !
• Writing you own echo program ?
• Remember how are command line options passed to a
  program ?
• main(int argc, char argv)
• argc number of command line arguments, including
  command itself
• argv the arguments

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Our own echo program

• include ltstdio.hgt
• main(int argc, char argv)
• //printf (Number of argumentsd\n,argc)
• //display all command line arguments, except
  the
• // command name itself
• for (int i1iltargci)
• printf (s ,argvi)

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Write a small program

• A program called pick
• Syntax pick arguments
• Function pick command presents the arguments one
  at a time and waits after each for a response.
  The output of pick is those argument selected by
  y response. Any other response causes the
  argument to be discarded.

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Some Experiments with echo

• Lets now use echo command to experiment with
  metacharacters
• echo Good morning, everyone!
• echo Good morning gt everyone
• echo lt message_file
• echo SHELL, echo PS1
• echo , echo .o
• If no matching is found, original string is
  passed on
• echo .pdf

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What if I want to display special characters ?

• What happens if we run
• echo Hello to the world gtgt !
• Use quotation to tell the shell to leave the
  metacharacters alone
• echo Hello to the world gtgt !
• echo Hello to the world gtgt !
• echo

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Different quotation

• Strongest one single quote character
• Echo
• Echo PS1
• Less strong quote double quote character
• Shell peeks inside for , and \
• echo Your PS1 is set to PS1
• To display Your PS1 is
• Echo Your \PS1 is set to PS1
• Backslash,\, protect a single character following
  it

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Echo multiple lines message

• When using quotes, shell knows the command is not
  finished when a newline is entered
• zhang_at_storm Echo echo "Hello,
• gt world,
• gt I can write my own echo."
• Hello,
• world,
• I can write my own echo.
• To enter a long command line
• zhang_at_storm Echo echo Hello world,\
• gt I can write my own echo.
• Hello world,I can write my own echo.
• zhang_at_storm Echo

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Practice Problems

• Create a file named ab without using any editor,
  and then delete the file
• Start a program named long_job in the background,
  sleep for 20 seconds and then check if the
  long_job process is still running or not
• Hint ps command lists all current processes
• grep ltsome_stringgt file search for the given
  string in the file or standard input

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Our first shell script

• How to find the ten largest file under your
  directory ?
• Commands to use ?
• Save the command sequence into a file
• Run the file script using
• bash 10largest
• bash lt10largest

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Script 10largest

• ls -Rl grep - sort -k 5 -nr head -10

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Run script like a command

• Would be nicer if the command is invoked same way
  as other commands
• Default permission for a new created file is
• zhang_at_storm uc cat gtdd
• echo Hello World !
• zhang_at_storm uc ls -l dd
• -rw-r--r-- 1 zhang staff 19 2008-01-24 2144 dd

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Change file permission

• Make the file executable
• chmod x 10largest
• all users now have the right to execute the file
• chmod ux 10largest
• the owner can execute the file
• Remember file command ?
• What will file 10largest return ?
• One can add an initial comment line
• !/bin/bash

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Pass argument to script

• 10largest script does not take arguments
• Would be nice if we can specify the directory to
  check, i.e., not always check current directory
• i.e. 10largest path_name
• We need to access the argument, and call commands
  with according directory
• ls -Rl path_name grep - sort -k 5 -nr
  head -10

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Shell parameter variables

• If your script is invoked with parameters, shell
  set the following variables
• the number of parameters
• 0 the command/script name
• 1 the first parameter given to the script
• 2,
• ,_at_ the list of all parameters
• Example 10largest2

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Script 10largest2

• echo
• echo 1
• echo 0
• ls -Rl 1 grep - sort -k 5 -nr head -10
• What if we call 10largest2 without argument ?

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Say we want to generate arguments

• We have used filename expansion (,? etc) as a
  way to generate arguments
• One can use a command to generate the argument
• echo Welcome, who am i. The time is date.
• Write to all students
• mail cat student_account.txt lt SampleScript
• What happens if we enter date on command line ?

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Use pick to choose from the list

• Remember our pick program ?
• Say we want to remove all files in a directory,
  but want to make sure before we delete each file
• rm pick
• mail pick \cat mailinglist lt sampleCode

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Summary

• Shell command line syntax
• Shell special characters, metacharacters
• Write our own echo command
• Use echo to experiment with metacharacters
• Quotation to protect metacharacters
• Our first shell script
• Passing arguments to shell script
• Generate arguments by running a command