Introduction to Bash Scripting

• Why Bash Scripting? (Bash)

  • Bash stands for ‘Bourne Again Shell’
  • Developed in the 80’s but a very popular shell today. Default in many Unix systems, Macs
  • AWS, Google, Microsoft all have CLI’s to their products
  • Bash Scripting helps in ease of execution of shell commands (no need to copy-paste every time!)
  • Powerful programming constructs

• Shell Commmands Refresher

  Some important shell commands:

  • (e)grep filters input based on regex pattern matching
  • catconcatenates file contents line-by-line
  • tail \ head give only the last -n (a flag) lines
  • wc does a word or line count (with flags -w -l)
  • sed does pattern-matched stringn replacement

• A reminder of REGEX

  ‘Regex’ or regular expressions are a vital skill for Bash scripting.

  You will often need to filter files, data within files, match arguements and a variety of otheruses. It is worth revisting this.

  To test your regex you can use helpful sites like regex101.com

• Some Shell Practice

  Let’s revise some shell commands in an example.

  Consider a text fiel fruits.txt with 3 lines of data:

    banana
    apple
    carrot
  

  If we ran grep `a` fruits.txt we would return:

    banana
    apple
    carrot
  

  But fi we ran grep `p` fruits.txt we would return:

    apple
  

  Recall that square parentheses are a matching set such as [eyfc]. Using ^ makes this an inverset set (not these letters/numbers)

  So we could run grep '[pc]` fruits.txt we would return:

    apple
    carrot
  

  You have likely used ‘pipes’ before in terminal. If we had many many fruits in our life we could use sort | uniq -c

  • The first will sort alphabetically, the second will do a count
  • If we wanted the top n fruits we could then pipe to wc-l and use head

      cat new_fruits.txt | sort | uniq -c | head -n 3
    

      14 apple
      13 banana
      12 carrot
    

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First Bash Script

• Bash Script Anatomy

  A Bash Script has a few key defining features:

  • It usually begins with #!/usr/bash (on itsown line)
    • So your interpreter knows it is a Bash script and to use Bash located in /usr/bash
    • This could be a different path if you installed Bash somewhere else such as /bin/bash/ (type which bash to check)
  • Middle lines contain code
    • This may be line-by-line commands or programming constructs

• Bash script anatomry

  To save and run:

  • It has a file extension .sh
    • Technically not needed if first line has the she-bang and path to Bash (#!/usr/bash), but a convention
  • Can be run in the termina using bash script_name.sh
    • Or if you have mentioned first line (#!/usr/bash) you can simply run using ./script_name.sh

• Bash Script Example

  An example of a full script (called eg.sh) is:

    #!/usr/bash
    echo "Hello world"
    echo "Goodbye world"
  

  Could be run with the command ./eg.sh and woudl output:

    Hello world
    Goodbye world
  

• Bash and Shell Commands

  Each line of your Bash Script can be a shell command.

  Therefore, you can also include pipes in your Bash scripts.

  Consider a text file (animals.txt)

        magpie, bird
        emu, bird
        kangaroo, marsupial
        wallaby, marsupial
        shark, fish
  

  We want to count animals in each group

  In shell you could write a chained command in the terminal. Let’s instead put that into a script (group.sh)

    #!/usr/bash
    cat animals.txt | cut -d " " -f 2 | sort | uniq -c
  

  Now (after saving the script) runnign bash group.sh causes:

    2 bird
    1 fish
    2 marsupial
  

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Standard streams & arguments

• STDIN-STDOUT-STDERR

  In Bash scripting, there are three ‘streams’ for your program:

  • STDIN (standard input). A stream of data into the program
  • STDOUT (standard output). A stream of data out of the program
  • STDERR (standard error). Errors in your program

  By default, these streams will come from and write out to the terminal.

  Though you may see 2> /dev/null in script calls; redirecting STDERR to be deleted. (1> /dev/null) would be STDOUT

• STDIN example

  Consider a text file (sports.txt) with 3 lines of data.

    football
    basketball
    swiming
  

  The cat sport.txt > new_sports.txt command is an example of taking data from the file and writing STDOUT to anew file. See what happends if you cat new_sports.txt`

    football
    basketball
    swimming
  

• STDIN v ARGV

  A key concept in Bash scripting is arguments

  Bash scripts can take arguments to be used inside by adding aspace after the script execution call

  • ARGV is the array of all the arguments given to the program
  • Each argument can be accessed via the $ notation. The first as $1, the second as $2 etc.
  • $@ and $* give all the arguments in ARGV
  • $# gives the length (number) of arguments

• ARGV example

  Consider an example script (args.sh):

        #!/usr/bash
        echo $1
        echo $2
        echo $@
        echo "There are " $# "arguments"
  

• Running the ARGV example

  Now running bash args.sh one two three four five

    one
    two
    one two three four five
    There are 5 arugments