1. Shells

2. shell basics

3. Shells
A program that is an interface between a user at a terminal and the computers resouces
The terminal may be real or an emulator
The shell is not a programming language
However, most shells do support scripting
The shell may:
Process commands from a terminal
a specific action
Process scripts
list of commands, typically in a file
Shells may be CLI or GUI

4. Shells Command Processor Commands typed on a terminal
Executed by the shell
Boune family
C family
many others…
Note: examples will center on the bash shell
bash – Bourne Again SHell
Shells do vary on implementation of some features

5. Shell preliminaries

6. Pattern Matching: Wild Cards
Wild Card Examples
Replace parts of names with arbitrary matching character(s)
Combination of known and unspecifed characters
?, *, […], {…}
Wild Card
Matches *
any number of chars including none ?
exactly one character
[aml]
single character from list
– either a, m, or l in this case
[a-l]
single character – in the range a to l
[!a-l]
single character – not in the range a to l
{ pat1, pat2…}
matches specifed patterns

7. Standard Files: Input/Ouput/Error
Standard input (std in)
Where the data is coming from
File
Stream
Usually the keyboard or terminal
Standard output (std out)
Where the data is going to
Usually the display or terminal
Standard error (std err)
Where error messages are sent
Default is same as Standard output

8. Data Sources/Redirection
Important: In Unix/Linux everything is a file
Terminal
Keyboard - source
Screen or printer – sink
File redirection
> and >>
Send the standard out data to a file
<
Receive the standard in data from a file
Piping |
Send the standard output of one program to the input of another
Different utilities and programs will handle defaults for std in and std out differently

9. Shell Basics

10. Shell Example – wc command
word count
reads the input and counts and reports the number of:
lines
words
characters
Examples of data source and sink using wc:
No parameters: data will flow from std in:
wc This is a line of data This is another <Ctrl-d>
File as a parameter
wc /dir/file /dir/file
Note it has the filename in the output
Can work on a list of files
Redirection from a file
wc < /dir/file
Piped from another program
ls | wc

11. File Descriptor Standard files have numeric representations Example:
0 – Standard input
1 – Standard output
2 – Standard error
Example:
#cat bad.file cat: bad.file: No such file or directory #cat bad.file 2> err.msg #wc err.msg err.msg #more err.msg cat: bad.file: No such file or directory

12. Problem: want std out and std err to go to same destination
Example: file1 exists, file2 does not
#cat file1 file2 file 1 data … cat: file2: No such file or directory
redirect std out
#cat file1 file2 1> report cat: file2: No such file or directory #more report file 1 data …
redirect std err
#cat file1 file2 2> report file 1 data #less report cat: file2: No such file or directory
cannot consistently redirect both streams to the same file, need another method
Can use >> for one or replicating descriptors
May show order differently or not work at all

13. Replicating Descriptors
1>&2
send std out to destination of std err
2>&1
send std err to destination of std out
redirect both std err and std out
#cat file1 file2 2> report 1>&2 #less report file 1 data cat: file2: No such file or directory

14. Special files /dev/null /dev/tty Note: “bit bucket”
your default terminal
useful when different users log on
everyone can use /dev/tty as their terminal
Note:
Remember: everything in Unix/Linix is a file!
/dev contains most of the machines devices

15. Pipes
Send output of one command as the input to another command or program
Examples
#ls | wc
Sent the short directory listing to wc
#ls –l | wc
Sent the long directory listing to wc

16. Tee Sends data to std out and a file Example: Not part of the shell
#who | tee users.txt ajk tty :14 (:0) ajk pts/ :45 (:0.0) #cat users.txt ajk tty :14 (:0) ajk pts/ :45 (:0.0)

17. Resume 1/11

18. Command substitution
Command arguments can be obtained or substituted from the std out of a program:
Use backticks ` to denote
That is not the single quote ‘ or the fancy Word quotes
AKA Accent Grave
Examples:
#echo This year is `date +%Y` This year is 2013
#ls –la `cat filelist` ls: cannot access file2: No such file or directory ls: cannot access file4: No such file or directory -rw-r-r- 1 ajkombol ajkombol 14 Mar 3 16:34 file1 -rw-r-r- 1 ajkombol ajkombol 14 Mar 3 16:34 file3
#cat filelist file1 file2 file3 file4

19. Shell variables Can assign values to variables
myvar=myvalue
Important: no spaces!
Note: no $ is needed to assign
Case sensitive
Use by putting $ in front
$myvar
Example:
#ext=.sh #name=doit #compname=$name$ext #echo $compname doit.sh
Summary:
Set the variable without a $
Use the variable with a $

20. Scripts Intro

21. Scripts Intro Shells can execute groups of commands in a file
Script basics:
They have basic control sequences
e.g. conditionals and looping
if, for, while
They may contain commands
They must have execute permission

22. Environment preview
Using the shell to modify your runtime environment

23. Shell variables - Environment
Two shell variable types:
Environment
Used by user(s) in general
PATH
HOME
SHELL
Etc…
Note:
By convention environment variables are UPPER CASE
Valid for who is “logged on”
Local
Used for specific purposes
Typically available to current process only
Case sensitive

24. Environment “Default” variables the shell uses Who is the current user
What is the current path
What is the current home directory
What do you have as a cursor
What is the default shell for this user …

25. Local Variables Easy to create for temporary use Restricted in scope
my_var=value
Restricted in scope
Usually the current sub-shell only

26. Environment Variables
To create an Environmental Variable
export ENV_VAR
Can apply value at creation or later
Good for creating a set of common definitions
By convention Environmental Variables are UPPER_CASE

27. customizing the environment
Using the shell

28. Setting the default shell
What shell is set as your default?
echo $SHELL
Returns something like:
/bin/bash -or-
/usr/bin/bash
Depends on distro
Popular shells
Bash
csh
korn
Bourne (sh)
Note: many systems link sh to dash

29. Environmental Variables
Available in the user’s total environment
Sub-shells
Scripts they run
Editors
Mail commands
etc.
Note: when you start a new terminal the ENV is reset
Local variables
Typically only available to the current process
set
displays all variables in the current shell
env
shows only environment variables

30. export Turns a local variable into an environmental variable Form:
Basically makes a variable visible to all child processes
Needs only to be done once for a session
Needs to be redone every time a new session is started
Form:
export ENV_VAR

31. Common Environment Variables
Significance
HOME
users home directory
PATH
list of directories to search for a command
LOGNAME
login name of user
USER
MAIL
absolute pathname to users mail file
MAILCHECK
how long to wait before checking for new mail
TERM
type of terminal
PWD
absolute name of users current directory
PS1
primary prompt string
PS2
secondary prompt string
SHELL
users login shell

32. Aliases Use alternate names "Shorthand" for commonly used commands
#alias vi='vim'
will always run vim when vi is typed
can run vi with \vi
Assign common options
#alias cp="cp -i"
will change cp to always be interactive
Asks if duplicate names found
#alias cp
will return the current assignment
#unalias cp
will get rid of the alias

33. Command history Allows recalling previous commands
Shows all the previous commands with event numbers
holds about 500 events
Will vary by distro
#history n
shows last n command
#!n
executes event n
#!n:p
prints event n (does not execute)

34. Tilde ~ Shorthand for the current users home directory Notes: #cd ~
changes to current user’s home directory
#cd ~userid
changes to userid’s home directory

35. Using set By default, set shows all variables Interesting options
#set –o noclobber
prevents accidental overwriting of existing files with redirection (the > and >> symbols)
#set –o ignoreeof
prevents accidental termination of script (<ctrl>d)
#set –o notify
allows completed background jobs to notify immediately when done

36. Initialization scripts
login script
runs once at log in
varies by distro
.bash_profile
.profile
.bash_login
usually in users home directory
rc script
runs every time an interactive sub-shell is created
varies by distro, usually has rc in its name
.bashrc
Used to customize the users environment
The prompt is usually set in one of these