Introduction to Shell Scripting

Shell Scripting Basics

What is a script?

• Script: list of commands interpreted by a scripting language
• Commands can be entered interactively or listed in a text file
• Scripting languages are interpreted at runtime

• Scripting is slower to run, but faster to develop

  What is a script used for?

• Widely used to automate processes
• ETL jobs, file backups and archiving, system admin

• Used for application integration, plug-in development, web apps, and many other tasks

  Shell scripts and the ‘shebang’

• Shell script – executable text file with an interpreter directive

• Aka ‘shebang’ directive

  #!interpreter [optional-arg]
  

• ‘interpreter’ – path to an executable program
• ‘optional-arg’ – single argument string

Example – ‘shebang’ directives

Shell script directive:

  #!/bin/sh
  #!/bin/bash

Python script directive:

  #!/usr/bin/env python3

Filters, Pipes, and Variables

Pipes and filters:

Filters are shell commands, which:

• Take input from standard input
• Send output to standard output
• Transform input data into output data
• Examples are wc, cat, more, head, sort, …

• Filters can be chained together

  Pipe command – |

• For chaining filter commands

  commmand1 | command2
  

• Output of command 1 is input of command 2
• Pipe stands for pipeline

Shell variables:

• Scope limited to shell

• Set – list all shell variables

  Defining shell variables:

  var_name=value
  

• No spaces around =

  unset var_name
  

• deletes var_name

Environment Variables:

• Extended scope

  export var_name
  

• env — list all environment variables

Useful Features of the Bash Shell

Metacharacters

• # — precedes a comment
• ; — command separator
• * — filename expansion wildcard
• ? — single character wildcard in filename expansion

Quoting

• \ — escape special character interpretation
• "" — interpret literally, but evaluate meta-characters
• '' — interpret literally

I/O redirection

Input/Output, or I/O redirection, refers to a set of features used for redirecting

• > — Redirect output to file
• >> — Append output to a file
• 2> — Redirect standard error to a file
• 2>> — Append standard error to a file
• < — Redirect file contents to standard input

Command substitution

• Replace command with its output

  $(command) or `command`
  

• Store output of pwd command in here:

Command line arguments

• Program arguments specified on the command line

• A way to pass arguments to a shell script

  ./MyBashScript.sh arg1 arg2
  

Batch vs. concurrent modes

Bath mode:

• Commands run sequentially

  command1; command2
  

  Concurrent mode:

• Commands run in parallel

  command1 & command2
  

Scheduling Jobs using Cron

Job scheduling

• Schedule jobs to run automatically at certain times
  • Load script at midnight every night
  • Backup script to run every Sunday at 2 AM

• Cron allows you to automate such tasks

  What are cron, crond, and crontab?

• Cron is a service that runs jobs
• Crond interprets ‘crontab files’ and submits jobs to cron
• A crontab is a table of jobs and schedule data
• Crontab command invokes text editor to edit a crontab file

Scheduling cron jobs with crontab

Viewing and Removing cron jobs