Shells. A program that is an interface between a user at a terminal and the computers resouces...
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Transcript of Shells. A program that is an interface between a user at a terminal and the computers resouces...
Shells
shell basics
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 (a specific action)▫Process scripts (list of commands)
•Shells may be CLI or GUI
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
Shell preliminaries
Pattern Matching: Wild CardsFilenames 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
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 File Stream
▫ Usually the display or terminal• Standard error (std err)
▫ Where error messages are sent File Stream
▫ Usually the display or terminal Default is same as Standard output
Data Sources/Redirection• Important: In Unix/Linux everything is a file• Terminal
▫ Keyboard - source▫ Screen or printer – sink
• File redirection ▫ > and >>
Send the standard data to a file▫ <
Receive the standard data from a file• Piping
▫ | Send the output of one program to the input of another
• Different utilities and programs will handle defaults for std in and std out differently
Resume 8/27
Shell Basics
Shell Example – wc command• wc
▫ word count reads the input and counts and reports the number of:
lines words characters
http://www.unix.com/man-page/posix/1posix/wc/ • Examples of data source and sink using wc:
▫ No parameters: data will flow from std in: wcThis is a line of dataThis is another<ctrl-d>
2 9 34▫ File as a parameter
wc /dir/file8 50 1084 /dir/file
Note it has the filename in the output Can work on a list of files
▫ Redirection from a file wc < /dir/file
8 50 1084▫ Piped from another program
ls | wc 6 6 60
File Descriptor• Standard files have numeric representations
▫0 – Standard input▫1 – Standard output▫2 – Standard error
• Example:▫#cat bad.filecat: bad.file: No such file or directory#cat bad.file 2> err.msg#wc err.msg 1 7 41 err.msg#more err.msgcat: bad.file: No such file or directory
Problem: want std out and std err to go to same destination
• Example: file1 exists, file2 does not▫ #cat file1 file2file 1 data cat: file2: No such file or directory
• redirect std out▫ #cat file1 file2 1> reportcat: file2: No such file or directory#more reportfile 1 data
• redirect std err▫ #cat file1 file2 2> reportfile 1 data#less reportcat: file2: No such file or directory
• cannot consistanly 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
Replicating Descriptors
•1>&2▫send std out to destination of std err
•2>&1▫send std err to destination of std out
•redirect std err, std out▫#cat file1 file2 2> report 1>&2#less reportfile 1 data cat: file2: No such file or directory
Special files
•/dev/null▫“bit bucket”
•/dev/tty▫your default terminal▫useful when different users log on▫everyone can use /dev/tty as their terminal▫Remember: everything in Unix/Linix is a
file!
Pipes
•Send output of one command as the input to another command or program
•Examples▫#ls | wc
31 32 530 Sent the short directory listing to wc
▫#ls –l | wc32 282 2121
Sent the long directory listing to wc
Tee
•Sends data to std out and a file▫Not part of the shell
•Example:▫#who | tee users.txtajk tty72013-02-16 22:14 (:0)ajk pts/0 2013-03-01 08:45 (:0.0)#cat users.txtajk tty72013-02-16 22:14 (:0)ajk pts/0 2013-03-01 08:45 (:0.0)
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 ' AKA Accent Gravé
• Examples:▫ #echo This year is `date +%Y`This year is 2013
▫ #ls –la `cat filelist`ls: cannot access file2: No such file or directoryls: 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 filelistfile1 file2file3 file4
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 $compnamedoit.sh
Scripts Intro
Scripts Intro
•Shells can execute groups of commands in a file▫Scripts
•Script basics▫They have basic control sequences
e.g. conditionals and looping if, for, while
▫They may contain commands▫They must have execute permission
Environment previewUsing the shell to modify your runtime environment
Shell variables - Environment• Two shell variable types:
▫Environment Used by a user in general
PATH HOME SHELL Etc…
Note: By convention environment variables are UPPER CASE
▫Local Used for specific purposes
Typically available to current process only
• Case sensitive
Local Variables
•Easy to create for temporary use▫my_var=value
•Restricted in scope
Environment Variables
•Export to create an Environmental Variable▫export ENV_VAR
•Can apply value at creation or later•Good for creating a set of common
definitions
customizing the environmentUsing the shell
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
Environmental Variables• Available in the user’s total environment
▫Sub-shells Scripts they run Editors Mail commands etc.
• Local variables▫Only available to the current process
• set▫displays all variables in the current shell
• env▫shows only environment variables
export
•Turns a local variable into an environmental variable▫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
Common Environment VariablesVariable Significance
HOME users home directory
PATH list of directories to search for a command
LOGNAME login name of user
USER login name of 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
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
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)
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
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
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