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 from a terminal a specific action Process scripts list of commands, typically in a file 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 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

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

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

Shell Basics

Shell Example – wc command 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: wc This is a line of data This is another <Ctrl-d> 2 9 34 File as a parameter wc /dir/file 8 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 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 1 7 41 err.msg #more err.msg cat: 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 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

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

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

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 | wc 32 282 2121 Sent the long directory listing to wc

Tee Sends data to std out and a file Example: Not part of the shell #who | tee users.txt ajk tty7 2013-02-16 22:14 (:0) ajk pts/0 2013-03-01 08:45 (:0.0) #cat users.txt ajk tty7 2013-02-16 22:14 (:0) ajk pts/0 2013-03-01 08:45 (:0.0)

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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

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 $

Scripts Intro

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

Environment preview Using the shell to modify your runtime environment

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

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 …

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

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

customizing the environment Using 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 (sh) Note: many systems link sh to dash

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

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

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

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