ops102_8_1.pdf

Bash Scripting OPS102 Week 8 Class 1 Chris Tyler/John Sellens July 2, 2024 Seneca Polytechnic Outline Script Basics Bash Details Summary OPS102 W8C1 - Bash Scripting 1/27 Script Basics What is a Script? A shell script is a simple computer program which is interpre...

Bash Scripting OPS102 Week 8 Class 1 Chris Tyler/John Sellens July 2, 2024 Seneca Polytechnic Outline Script Basics Bash Details Summary OPS102 W8C1 - Bash Scripting 1/27 Script Basics What is a Script? A shell script is a simple computer program which is interpreted by an operating system shell. Scripts are used to automate procedures that could be manually performed from the command line. OPS102 W8C1 - Bash Scripting 2/27 How will scripts save hours of my life? If you’re developing a computer program which requires 25 steps to build and test, and you’re going to iterate through the build and test process 100 times, you can: Perform 2500 steps manually – a lot of work, and error-prone; or Write a script containing those 25 steps and then execute it each time you want to build and test your program (100 times) You can even set things up so the script is executed automatically when certain conditions are met – such as saving a change to your source code! OPS102 W8C1 - Bash Scripting 3/27 Basic Requirements for Shell Scripts 1. Create a text file containing shell commands. 2. Tell the operating system which shell to use to execute the commands. 3. Ensure that the script file has the appropriate permissions. OPS102 W8C1 - Bash Scripting 4/27 1. Create a file containing shell commands Use any text editor Use the same commands that you would type at the command-line Save the file OPS102 W8C1 - Bash Scripting 5/27 2. Tell the operating system which shell to use Add a “shebang” line to the start of the file First character is a sharp: # Second character is a bang: ! The rest of the line is the absolute pathname of the shell / interpreter #!/bin/bash The name “shebang” comes from “sharp” and “bang” Also known as a “hashbang” line The #! characters form a “magic number” which the kernel notices And runs the command: /bin/bash name-of-file The # character causes the shell to interpret the line as a comment, and ignore it OPS102 W8C1 - Bash Scripting 6/27 3. Set the correct permissions on the file The kernel needs execute permission to analyze the shebang line and execute the correct shell/interpreter. The shell needs read permission to read the contents of the file. Set these permissions with the chmod command for any user(s) that should be able to execute the file: chmod u+rx scriptname OPS102 W8C1 - Bash Scripting 7/27 Demo: Basic scripts Let’s write some simple scripts using commands that we know Scripts act like any other executable file, so we can type the script name as a command. However, unless the script is in a directory that is normally searched by the operating system, it won’t be able to find it. We’ll talk about how to adjust this later, but for now, we can use a pathname that includes a slash to execute a script we’ve written:./scriptname OPS102 W8C1 - Bash Scripting 8/27 Bash Details Setting Variables To set a variable: VARIABLE=VALUE Variable names start with a letter and can contain letters, numbers, and underscores. Case matters! A and a are different variables. Don’t put spaces on either side of the equal sign. Unlike assignments in most other languages. OPS102 W8C1 - Bash Scripting 9/27 Accessing Variables To use a variable value in a command, precede it with a dollar sign: $VARIABLE You can use a variable anywhere in a command: Arguments Command name The value of the variable is substituted into the command. OPS102 W8C1 - Bash Scripting 10/27 Variables: A Simple Example $ what=World $ echo Hello $what Hello World OPS102 W8C1 - Bash Scripting 11/27 Word Splitting / Tokenization The shell uses certain separator characters (whitespace by default) to split commands into words. For example, spaces are used to break this line into a three parts (or tokens): a command and two arguments $ ls –l filename But this means that arguments, such as filenames, that contain spaces will be interpreted as multiple arguments: $ ls –l file one OPS102 W8C1 - Bash Scripting 12/27 Preventing Word Splitting with Quoting Quoting prevents words from being split. It’s needed whenever we’re dealing with text that contains separators (such as space or tabs). You can also use quoting to hide other special characters from the shell, such as the file globbing characters: * ? [ ] OPS102 W8C1 - Bash Scripting 13/27 Types of Quotes: Single vs Double Single or double quote characters may be used to quote strings: B='Hello World' A="Hello World" When double quotes are used, variables will be expanded inside the quotes. When single quotes are used, variables will not be expanded inside the quotes. OPS102 W8C1 - Bash Scripting 14/27 Quoting: Examples $ what="World" $ echo "Hello $what" Hello World $ echo 'Hello $what' Hello $what OPS102 W8C1 - Bash Scripting 15/27 Quoting: More Examples $ what="World" $ message="Hello $what" $ what="There" $ echo $message Hello World $ echo "$message" Hello World $ echo '$message' Hello $message Note that the variable substitution of $what happened during the assignment to message and not when message was used. OPS102 W8C1 - Bash Scripting 16/27 Quoting: One argument vs. Multiple When a string contains a separator such as a space, and it is unquoted, the shell will interpret it as multiple words (tokens). When used as an argument, this will be interpreted as multiple arguments: $ touch "new file" $ ls -l new file ls: cannot access 'new': No such file or directory ls: cannot access 'file': No such file or directory $ ls -l "new file" -rw-r--r--. 1 chris chris 0 Jun 18 22:47 new file OPS102 W8C1 - Bash Scripting 17/27 Quoting: One argument vs. Multiple You should always double quote variables that may contain a space in their value when using them as command arguments. There are one or two slightly obscure exceptions to this rule. This is especially true for filenames -– you never know when a user is going to put a space (or a special character) in a filename! Many scripts work fine with opaque filenames (those containing no whitespace) but fail with non-opaque filenames. OPS102 W8C1 - Bash Scripting 18/27 Quoting: Backslashes (Escaping) A backslash character outside of quotes or inside double quotes instructs the shell to ignore any special meaning that the following character may have. $ touch "new file" $ ls -l new file -rw-r--r--. 1 chris chris 0 Jun 18 22:49 new file $ echo "This string contains a \"quoted\" string" This string contains a "quoted" string $ A=Testing $ echo " \$A" $A OPS102 W8C1 - Bash Scripting 19/27 Shell Variables vs Environment Variables By default, a variable is local to the shell in which it is running. Somewhat similar to local variables in C functions. You can export variables to make them environment variables. That means that they are passed to programs (child processes) that are executed by the shell. Environment variables are commonly used to pass configuration information to programs and to configure how programs operate. Environment variables are used by all processes, not just the shell! By convention, we use UPPERCASE to name environment variables. OPS102 W8C1 - Bash Scripting 20/27 Common Environment Variables Environment Variable Purpose EDITOR Name of the default text editor (often /usr/bin/nano) PATH A colon-separated list of directories that will be searched when looking for a command LANG The default language – used to select message translations as well as number, currency, date, and calendar formats. HOME The user’s home directory -– used for relative-to-home pathnames. OPS102 W8C1 - Bash Scripting 21/27 Viewing and Creating Environment Variables See all current environment variables and their values: $ printenv | less The set command shows all variables. Create an environment variable with export: $ X=500 $ export X $ export Y=123 OPS102 W8C1 - Bash Scripting 22/27 Example: PATH Environment Variable $ cat showdate #!/usr/bin/bash date $./showdate Sun 18 Jun 2023 11:20:18 PM EDT $ showdate bash: showdate: command not found $ echo $PATH /home/chris/bin:/usr/local/bin:/usr/bin:/bin $ PATH="$PATH:." $ showdate Sun 18 Jun 2023 11:20:45 PM EDT Note: Having. in your $PATH is a security risk. But is the default on matrix. OPS102 W8C1 - Bash Scripting 23/27 Example: LANG Environment Variable $ echo $LANG en_CA.UTF-8 $ date Sun 18 Jun 2034 11:32:27 PM EDT $ foobarbaz bash: foobarbaz: command not found $ LANG=fr_CA.UTF-8 $ date ven nov 3 01:45:08 EDT 2023 $ foobarbaz bash: foobarbaz: commande introuvable See also the LC_* environment variables: https://unix.stackexchange.com/questions/87745/what-does-lc-all-c-do OPS102 W8C1 - Bash Scripting 24/27 Example: HOME Environment Variable $ echo $HOME /home/chris $ echo ~ /home/chris $ HOME=/ $ echo ~ / $ ls ~ afs bin boot dev etc home lib lib64 lost+found media mnt opt proc root run sbin srv sys tmp usr var Note: changing $HOME can cause you problems. OPS102 W8C1 - Bash Scripting 25/27 Example: PS1 Local Shell Variable Change the shell prompt: $ PS1="Enter a command: " Enter a command: date Sun 18 Jun 2023 11:17:14 PM EDT Enter a command: PS1="[\u@\h \W]\$ " [chris@toronto ~]$ PS1="$ " $ Note: PS1 looks like an environment varible because it’s uppercase, but it isn’t. OPS102 W8C1 - Bash Scripting 26/27 Summary Summary Scripts are handy, and easy to create – yay text files! The “shebang” line is a “magic number” that guides the kernel on how to execute a script. Variables are handy – local and environment. Quoting and backslash escaping hide special characters and whitespace. OPS102 W8C1 - Bash Scripting 27/27