Workbook 6 – Part 2 The Bash Shell

1. Workbook 6 – Part 2 The Bash Shell RH030 Linux Computing Essentials

2. Workbook 6 Objectives • This workbook is all about the shell. Last week we discussed • What is a Shell • What a shell is and what it does • How subshells is and how it works • Overviewed shell features • Shell variables • Should understand difference of Local verse Environmental variables • Creating new shell variables • Exporting variables • Identifying common shell environment variables • $LOGNAME, $SHELL,$HOME, $PATH • Manipulating common shell environment variables • ~/.bashrc Linux+ Guide to Linux Certification, 2e

3. Workbook 6 Objectives This week • Shell feature = Command Line expansion • Startup & Login environment files. • global & local environment files. • Sequence of the startup environmental files • Use of each startup environmental file • Manipulating common shell environment variables permanently • Creating permanent aliases/variables Linux+ Guide to Linux Certification, 2e

4. Chapter 4 – Command line expansion • Key Concepts • The bash shell expands certain command line metacharacters before interpreting the command. • Tilde expansion expands tokens that begin with a tilde (~) to users home directories. • Brace expansion expands tokens with braces ({}) into multiple words, each of which contains a single word from the specified list. • Command substitution expands text enclosed within backticks (``) or "dollar parenthesis" ($()) into the output produced by the enclosed command. • Double quotes ("..." ), single quotes ('...'), and the backslash character can be used to protect characters from being expanded by the shell. Linux+ Guide to Linux Certification, 2e

5. Using Command Line Expansion • Expansion Area Syntax Expands To History ! Refer to a previous command line Brace {} Create this specified text Tilde ~ Refer’s to a user's home directory Variable $, ${...} Shell and environment variables Arithmetic $((...)) Numeric calculation Command Substitution ` ...`, Allows command to run in subshell Pathname *, ?, [^...] Matchs to filenames in filesystem Linux+ Guide to Linux Certification, 2e

6. Brace • Quick way to create items Create this specified text touch file{01,02,03,04} • [student@station student]$ ls file01 file02 file03 file04 • Creating a hierarchy structure [student@station student]$ mkdir chap{01,02,03,04}/{html,text} Linux+ Guide to Linux Certification, 2e

7. It can get quite complicated • $touch soap/episode{1,2,3,4}.{draft,final}.{txt,pdf} • Will produce the following directory listing. • [student@station student]$ ls soap/episode1 episode2.draft.pdf episode3.draft.pdf episode4.draft.pdfepisode1.draft.txt episode2.draft.txt episode3.draft.txt episode4.draft.txt episode1.final.pdf episode2.final.pdf episode3.final.pdf episode4.final.pdf episode1.final.txt episode2.final.txt episode3.final.txt episode4.final.txt Linux+ Guide to Linux Certification, 2e

8. Pathname Expansion • Character Matching * 0 or more characters ? exactly one characters [...] exactly one of the included characters [^...] exactly one of the excluded characters Linux+ Guide to Linux Certification, 2e

9. Different Quotes do different things Linux+ Guide to Linux Certification, 2e

10. Using quota’s • Sometime the entire line needs to be enclosed in quota’s • Double quotes tell the shell to: ( “ ) • Display the value of this variable used. LOGIN=John echo “Welcome $LOGIN” Welcome John • Single quotes tell the shell to: ( ‘ ) • Display the name of this variable used LOGIN=John echo ‘Welcome $LOGIN’ Welcome $LOGIN • Back quotes ( ` ) tell the shell to run text this as a command echo “ Welcome $LOGIN today is `date` ” Linux+ Guide to Linux Certification, 2e

11. Command Substitution • The "old school" syntax for command substitution is to encapsulate the command in `back ticks` • The more modern syntax supported by the bash shell is similar to arithmetic expansion, but with only one set of parentheses: $(subcommand) • [prince@station prince]$ mkdir reports.$(date +%d%b%Y) Linux+ Guide to Linux Certification, 2e

12. Quoting • Put example of quotes from 1st ppt here Linux+ Guide to Linux Certification, 2e

13. Using quota’s • Here with no quotes, bash interprets the > and < characters as requests to redirect the command's output (and input). $ echo <pre> little red $CAR </pre> $ -bash: syntax error near unexpected token `newline' • Here the double quotes protected the < and > characters. • The dollar sign, however, is interpreted as a marker for a variable. $ echo “<pre> little red $CAR <pre>“ $ <pre>little red corvette </pre> • Here the single quotes stop the variable expansion $ echo '<pre>little red $CAR</pre>' $ <pre>little red $CAR</pre> Linux+ Guide to Linux Certification, 2e

14. Escape Characters • Shell expansions occur before the command is run. • Occasionally, some commands use arguments that contain characters in them that are special to the bash shell. • Meaning that for bash they have some other meaning. • Bash needs to be told NOT to read them as special characters. but to use them in the current syntax. \<special character> • A common example is the find command. find –name “*.chap” –exec rm {} \; Linux+ Guide to Linux Certification, 2e

15. Use of escape character • echo \<pre\>little red $CAR\</pre\> <pre>little red corvette</pre> • echo \<pre\>little red \$CAR\</pre\> <pre>little red $CAR</pre> Linux+ Guide to Linux Certification, 2e

16. Chapter 5 – Shell Customizations Key Concepts • The bash shell internally implements certain simple commands (internal Commands) • Which affect the shell's behavior. • These are referred to as builtin commands. • Shell aliases are established and examined with the alias command. • Shell aliases are removed with the unalias command. • The bash shell prompt can be customized using the PS1 variable. • Shell flags can can be set with the set -f command, and cleared with set +f. • Shell options are examined, set, and unset using the shopt command. Linux+ Guide to Linux Certification, 2e

17. Using alias’s • alias command: • Used to list all the available aliases. • Using aliases • Short alias runs a long command-line • Can create a single alias for multiple commands • But are only temporarily available unless you make them permanent. • Locally this is done using the .bashrc environment file. • Creating temporary alias’s • alias c=clear • alias c=“ clear;date;ls –l ” • alias ts="touch $(date +timestamp.%H:%M:%S)" • alias tsc="find . -newer" Linux+ Guide to Linux Certification, 2e

18. ~/.bashrc = local permanent aliases • Making aliases permanent • echo alias c=clear >> .bashrc • . ./.bashrc Linux+ Guide to Linux Certification, 2e

19. Remember - Last week we looked at variables There are 2 types of variables: • User-defined variables: • Custom variables • Created and defined by the local user • Environment variables: • Contain system information or properties that the system and programs both use in the running of the overall of the system and access regularly. • Such as your unique login id. Linux+ Guide to Linux Certification, 2e

20. Examples of Some Frequently Used Environmental Variables • Name Description • LOGNAME Sets the unique id by which this user will be identified • HOME Sets the directory which will be this users home directory. • PATH Specifies the directories that the shell is to look through to find an executable command. These directories are searched in the order of the path. • SHELL Sets your default shell. • HOSTNAME Sets the name of your system • MAIL Location of the users mailbox. • PS1 Sets how your prompt will be displayed. Linux+ Guide to Linux Certification, 2e

21. Working with Variables & their values • Displaying all currently available variables & their current values set • Displaying the value of a specific variable: • To view contents of a specified variable you must always use a $ sign infront of the variable name. • echo $HOME • Creating or Changing value of a variable: • To create a new variable name you use the following format. • <new-variable-name> equal sign (=) and new value Sheila=teacher • Double quotes should surround an entire string. Sheila= “Sheila is the teacher” Linux+ Guide to Linux Certification, 2e

22. Modifying existing variables • Customizing Your Prompt echo $PS1 “[\u@\h \W]$” Default setting PS1=“Hello World” echo $PS1 Hello World /home/sheila $pwd /home/sheila $ /home/sheila$ PS1= “This month is `date +%h ` @” This month is May @ The back quotes (`) are used to run it as a command instead of interpreting it literally as a string of text. Linux+ Guide to Linux Certification, 2e

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24. set verse shopt • These two builtin commands are used to configure the shell's behavior using shell options. • In other words they turn on functions. • One is the set command, which can also be used to modify the shell's behavior using (usually) single letter flags set –f set –o noclobber (c shell function) • The other is shopt, which is used to configure shell options. shopt –s cdspell shopt –u cdspell Linux+ Guide to Linux Certification, 2e

25. Chapter 6 – Sourcing Shell Scripts and Shell Initialization Key Concepts • Shell scripts are sourced with the source or . commands. • Shell scripts are all executed in the same shell that sources the script. • Bash shells can be either login or non-login shells. • Bash shells can be either interactive or non-interactive shells. • /etc/profile, files within the /etc/profile.d directory, and ~/.bash_profile are sourced on startup of every login shell. • /etc/bashrc and ~/.bashrc are sourced on startup of every shell. • ~/.bash_logout is sourced by every exiting login shell. Linux+ Guide to Linux Certification, 2e

26. source = run this script • [blondie@station blondie]$ cat prompts.script PS1="whadda you want? " #PS1="\a\u@\H \$(date --iso-8601) \t [\!] \$ " #PS1="[\u@\h \W]\$ " • In order to try out the first prompt, she sources the file prompts.script. [blondie@station blondie]$ source prompts.script whadda you want? • Can also use . ./prompts.script • Can also use bash ./prompts.script Linux+ Guide to Linux Certification, 2e

27. Environment Files • Environment files are used as part of the logon process • During login all users are assigned many default values. • Such as a default shell, path, home directory etc • All the different shells each use their own environment files. • When their shell is started it reads it’s own environment files. • There are 2 types of environment files. • Global –used when you login and given to everyone • Local – Also read when you login PLUS when you start a new subshell –used only to the local specific user. Linux+ Guide to Linux Certification, 2e

28. Login Process • Environment files are just : • Script configuration files • Some are run everytime you login. • There are 2 types: • Global environment files • /etc/profile • /etc/bashrc • local • Bash environment Files • ~/.bash_profile • ~/.bashrc • ~/.bash_logout Linux+ Guide to Linux Certification, 2e

29. Last week we discussed exporting variables • Exporting variables = Making the entire system aware of them. • It declares the existence of defined variables within the system. • A variable needs to be exported or declared to the entire system so that they are available for use. • Making them available to all subshells. export command = 2 uses • Used on it’s own • It will lists all the currently exported / declared available variables export teacher • Used with an existing variable it will export/declare that variable. • This then ensures that all sub-shells have access to this variable Linux+ Guide to Linux Certification, 2e

30. Items - you would export in Global Environment • Exports the login id you used to connect to the system with. • $LOGNAME for login name etc • Exports PATH for Default Command Path • $PATH sets default path of directories where the shell will look when a command is executed • Exports MAIL for Default Mailbox location • $MAIL sets default path of directories containing each specific users mailbox location. • Sets TERM Variable Default Terminal Type • screen and keyboard • Displays Contents of /etc/motd File • Holds any motd is ‘message of the day’ • Sets Default File Creation Permissions • sets umask values which determines the default permissions • Checks for Mail • Checks mail and will print a mail message upon login Linux+ Guide to Linux Certification, 2e

31. Order of the environment files • /etc/profile is read first • It contains the global variables. • It exports the global variables. • ~/.bash_profile is read next • It contains the local variables. • It exports the local variables. • ~/.bashrcis read next • It contains the local aliases. • Code within it causes it to read the /etc/bashrc if it exists. • /etc/bashrcis read last • It contains the global aliases. Linux+ Guide to Linux Certification, 2e

32. Note During login different shells use different Local Environment Files Linux+ Guide to Linux Certification, 2e

33. Summary • echo $? && || • set env export • $ PATH $HOME $SHELL $PS1 • alias set shopt • source . ./script1 bash script1 • /etc/profile /etc/bashrc • ~/.bash_profile ~/.bashrc ~/.bash_logout Linux+ Guide to Linux Certification, 2e