Shell Scripting Crash Course

Transcription

1 Shell Scripting Crash Course Travis Phillips JAX LUG

2 Overview What is a shell script What can I do with a shell script How to build shell scripts Syntax basics Basic useful shell commands Pipes and redirectors

3 Overview (Continued) Shell variables Getting user input Special script variables Inline command substitution Logic flow control Check if user is root Using functions for modulation of code

4 What is a shell script A shell script is simply a file It contains a series of several shell commands on it s own lines that the user could run via hand typing in the shell directly. Instead of having to type all of these commands, this provides a means to just invoke the file and each command would be run. Provides a means of programming just using what is already provided to us natively at the shell

5 What Can I Do With Them Why should I care? Scripts are useful for many things: Automating common task. Automating large task. Simplifying a chain of several commands. Some commands in Linux can become difficult to remember once you add in all the options and switches. Providing tools that ensure everyone is doing thing in a uniform manner. In short, ANYTHING YOU CAN DO WITH A LINUX SHELL Which is EVERYTHING!

6 Quick Warning On Automation Computers are very fast. Computers can process things in bulk. Computers can automate your mistake VERY QUICKLY AND IN BULK!!! Use caution when you decide to automate things and test it on a nonproduction machine.

7 A Quick Look At A Shell Script #!/bin/bash mkdir ~/test touch ~/test/test1 ~/test/test2 ~/test/test3 ls -l ~/test rm -rf ~/test/* rmdir ~/test

8 A Quick Look At A Shell Script Save it in a file such as test.sh Give it execute permissions chmod +x test.sh Finally, run it!./test.sh

9 Pound Is Your Friend Pound signs are used to place comments. Anything after the pound is a comment. Comments are ignored by the shell and are their for our purposes to comment code and leave notes. It is a good Idea to comment your code! In fact, ALWAYS comment your code!

10 Shebang As you may have noticed the first line was #!/bin/bash. A shebang is the #! part. It was followed with the path to the bash binary This tells the script that the correct binary to execute this with is /bin/bash. The bash shell. Useful to ensure the shell has the functionality you are requesting. MUST BE ON THE VERY FIRST LINE!!! This is so it knows it s not a comment since it starts with a pound sign.

11 Shell Basics Quick Guide of useful commands. ls used to List files and directories and return info on them. mkdir Makes a directory. cp Copies a file or directory mv Moves a file or directory. Can also be used to rename a file. echo takes input information and outputs it to the screen. Note: -e and -n are useful with this one.

12 Shell Basics sleep makes the shell pause and wait a given amount of seconds. cut A tool used to extract info from a given byte range or delimited field. grep A useful tool to locate string or regular expression matches. sed a stream editing tool that is useful for find and replace operations. Supports regular expressions. man the manual. Contains info on commands Always RTFM before asking questions to avoid being flamed.

13 Shell Basics Pipes -- Pipes are used to take one programs output and feed it into another program. Useful for stacking program functionality such as grep. Put that in your pipe and grep it! Hacksonville Slogan. Unix follows the idea that a tool should do one thing, one thing only, do that one thing well, but be able to play nicely with others. Example to view only TCP ports in netstat netstat -l grep ^tcp

14 Shell Basics Redirectors -- > < Redirectors are used to send stderr and/or stdout somewhere else such as a file. This can be useful for logging purposes. netstat > ~/netconnections.txt Or to silence a commands output if you wish to keep it suppressed. sudo apt-get install nmap > /dev/null

15 Shell Basics Escape characters -- \ Backslash is used to escape special characters we don t want the shell to interrupt and instead just treat as a string. Example would be redirectors or pipes for example. Also escape spaces with it as the shell uses spaces as a delimiter between arguments. Backslash is also escaped with a backslash. Think double negatives: \\

16 Working With Shell Variables Shell variables are a place to store data. Can only contain A-Z, 0-9, and underscores (_). By convention should be uppercase. Set Static using var_name=value Example: MYVAR=Stuff Once set, use in your script by putting a dollar sign. Example: echo $MYVAR

17 Working With Shell Variables You can also read values dynamically at runtime from the user using the (wait for it.) read command Example: read MYVAR This will read till the user hits enter and place the user input in the variable MYVAR.

18 Special Shell Variables to Be Aware of $0 Current script $[number] arguments passed to the script. Example: myscript.sh testing this out $0 would be myscript.sh $1 would be testing $2 would be this $3 would be out $4, $5, $6, etc would $# - Gets the total number of arguments passed to a script.

19 Special Shell Variables to Be Aware of $* - Passes in all of the arguments. This is useful for FOR loops. Example: myscript.sh test testing These are the same: echo $1 $2 echo $* $? Gets the error code (exit() status) of the last program executed. This is useful if you need to determine if a command was successful or not and respond differently depending on that.

20 Special Shell Variables to Be Aware of $$ - Gets the PID of the current shell. Since a shell script executes in the shell, that would be the PID of the script. $! Gets the PID of the last background process. Useful for managing timeout threads on background threads.

21 Special Shell Variables to Be Aware of $EUID Gets The EFFECTIVE UID number of the scripts execution. This is useful if you need to check that the user is running as root (or used sudo) or to check that a script is only executed by a application user.

22 Command Substitutions If we need to do a command substitution (that is, execute a command and drop it s output directly inline) we have two options. First is using $() $(command) Second is using backticks `command` Useful tool for setting a variable with the output of a command.

23 Logic Control using if The If command enables us to determine logic flow using a true or false statement. Terminates with fi command. else controls how it flows if it doesn t return true. elif to stack another if in the else statement.

24 If Compare Operators -eq : is equal to -ne : is not equal to -lt : is less than -gt : is greater than -le : is less than or equal to. -ge : greater than or equal to.

25 If Switches -s : file exists and is not empty -f : file exists and is not a directory -d : directory exist -x : file is executable -w : file is writable -r : file is readable

26 And Or Operators && : AND operator Requires both sides of it be true : OR operator Requires one or both sides of it to be true Values AND OR TRUE:TRUE True True TRUE:FALSE False True FALSE:FALSE False False

27 So Say We Wanted to Check If a User Was Root Check the UID. If it s 0, proceed. If not, throw error.

28 This in a Script Would Be #!/bin/bash if [ $EUID -eq "0" ]; then else fi echo "Yay!!! You are Root! WOOT!" echo "ERROR: SCRIPT MUST RUN AS ROOT!"

29 Can t Spell Functions Without FUN! Functions provide us a means of modulating our code. This makes the code easier to debug This makes the code more manageable This makes your code smaller This makes your code reusable! Helps to follow the coding practice of Don t Repeat yourself.

30 Can t Spell Functions Without FUN! Functions are created with a name. You later just call it by name and the block of code with in it will execute. Can take input parameters and return a value as well. Functions MUST BE CREATED BEFORE YOU USE THEM So rule of thumb: Keep your functions at the top of your script.

31 Example of When This is a Good Idea

32 Anatomy of a Shell Function

33 A Real World Example Syntax usage message. Make it a function. Why not? We will print it if we don t get arguments from the user Or if the arguments are invalid throw it there Or if we get -h or --help. since it may be a few lines echo statements, why not put it in a function call showusage()? Simple to update changes to it in one spot rather than 3 or 4 spots, 1 or a few may be missed if you have to do that.

34 Recap The Linux shell is awesome No I m not biased. A shell script is a file that has a stack of shell commands in it that will execute when invoked. Functions make code easier to manage. Comments should be used often. Only limitations are really a lack of creative thinking. If you haven t already, run the command man bash Read it. It is length but a lot of really good info lives there.

35 Q&A Time