DOCSLIB.ORG

Introduction to Linux Pt. 2

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text

Load more

INTRODUCTION TO LINUX PT2 Oliver Standford STOR-I, Lancaster University Introduction to Linux | Oliver Standford SHELL SCRIPTING Introduction to Linux | Oliver Standford Intro • SHELL • A program that interprets commands. • An intermediary between the user and system. • A programming language. • SCRIPTS • A Script is a shell program. • Easy to use tool for building applications by ‘gluing together’ different programs. • Well suited for repetitive tasks. Introduction to Linux | Oliver Standford Hello World #!/bin/bash echo "Hello World!" • #! /bin/bash - comment explaining which program to use. • echo “Hello World!” - making use of the echo program. • #!/bin/bash • tar -zcvf my-backup.tgz /home/guest00 • A very simple backup script • Create a tar backup of a user’s home directory. • NOT intended to be used. • Can use any text editor - nano, emacs, vim, gedit Introduction to Linux | Oliver Standford Executing a script • Execute your script. • bash your-script-name.sh • sh your-script-name.sh • ./your-script-name.sh – needs to be executable. • Set Executable permissions. • chmod +x your-script-name • chmod 755 your script name (owner - R,W,X, Group/Other - R,X) • Example. • chmod +x hello.sh • ./hello.sh Introduction to Linux | Oliver Standford Command Substitution • Command substation is to run a shell command store its output to a variable or display it back using the echo command. #!/bin/bash OF=my-backup-`date +%Y_%m_%d`.tgz tar -zcvf $OF /home/guest00 • A better backup script. • What is the function of: `date +%Y_%m_%d` • Why is it useful in this context? • For command substitution we use ` (backticks) not “” (quote). • Create a similar script. Introduction to Linux | Oliver Standford Variables • Scripts can use variables as in any programming language. • There are no data types. • Can contain a number, character or a string of characters. #!/bin/bash STR=“Hello!”; here=`pwd`; echo “$STR you are at $here” • What will the output of this script be? Introduction to Linux | Oliver Standford Passing Arguments bash myscript.sh arg1 arg2 arg3 echo There are $# arguments to $0: $* echo first argument: $1 echo second argument: $2 echo third argument: $3 • Arguments typed after the script name on a shell command line are passed to the script as variables. • $* - returns a single string containing all arguments. • $1,2 etc - referrers to a numbered argument. • $0 - referrers to the script name itself. • $# - referrers to the number of arguments in the command line. • What is the output of the above script? Introduction to Linux | Oliver Standford Setting variables interactively • Using the Read command. • Reads a line from standard input, and • Assigns each word to the corresponding shell variable. #!/bin/bash echo "What is your name?"; read MYNAME; echo "Hello $MYNAME - hope you’re well." Introduction to Linux | Oliver Standford Backup Script Revisited #!/bin/bash echo "Extension of files to back-up"; read extension; OF=my-backup-`date +%Y_%m_%d`.tgz tar -zcvf $OF `find ~/ -name "*.${extension}“ -print` • A more specialised backup script. • READ - asks the user what extension to backup. • FIND - searches for files matching that extension. Introduction to Linux | Oliver Standford Exercise • Create a backup script that has the following features. • When the script is run, the user will be prompted to specify the location of the files to be backed up. • The script will then ask the user where they want to save their backup. • The script will prompt the user to give the backup a name. • The script name will be appended with the date and time of the backup. • Modify the script so that the folder to be backed up is passed by an argument instead of prompting the user for the location. Introduction to Linux | Oliver Standford Conditionals • Decide whether or not to perform an action. • Decision taken by evaluating an expression. #!/bin/bash T1=“foo” T2="bar” if [ "$T1" = "$T2" ]; then echo expression evaluated as true else echo expression evaluated as false fi Introduction to Linux | Oliver Standford File test operators • -e file exists • -f file is a regular file (not a directory etc) • -s file is not zero size • -d file is a directory • -L file is a symbolic link • -r file has read permission • -w file has write permission • -x file has execute permission • f1 -nt f2 f1is newer than f2 • f1 -ot f2 f1 is older than f2 Introduction to Linux | Oliver Standford Other comparison operators Integer Comparison String Comparison -eq is equal to = is equal to -ne is not equal to != is not equal to -gt is greater than -z is null, zero length -ge is greater than or -n is not null equal to. -lt is less than -le is less than or equal to. Introduction to Linux | Oliver Standford For do Loops • Loops are used to perform actions over and over until a condition is met or until all data is has been processed. • The For Do loop iterates through a list, executing the body of the loop for each value in the list. for i in 0 1 2 3 4 5 ; do echo The number is $i done • The for loop assigns the value of each list item to the variable NUMBER. Introduction to Linux | Oliver Standford For loops and command substitution • For loops are meant to be used with Command Substitution. for FILE in `ls` ; do echo The file name is $FILE done • Display the contents of a directory. for i in `seq 1 10` ; do echo “i = $i”; done • Print out a list of numbers one to ten using seq command. Introduction to Linux | Oliver Standford While Loops • A while loop executes while a condition is true. • A simple while loop. #!/bin/bash i=0; while [ $i -lt 10 ]; do echo "i= $i"; let i=$i+1; done • While i is less than 10, loop through the body, adding 1 to i each time. Introduction to Linux | Oliver Standford Shell Arithmetic • Integer arithmetic can be performed using the built-in let, expr or bc commands. BIGNUM=1024 - Assigns number to variable. let RESULT=$BIGNUM/16 - divides 1024 by 16. RESULT=`expr $BIGNUM / 16` - divides 1024 by 16. RESULT=`echo “$BIGNUM /16” | bc` - divides 1024 by 16. let foo=$RANDOM%10; echo $foo - generates a random number between 1 and 10. Introduction to Linux | Oliver Standford Exercises • Write a simple shell script that takes a path of a directory as a command line argument and list all files and folders inside the given directory. • Write a script that asks for the user's age. If it is equal to or higher than 18, print a message saying that this user is allowed to drink alcohol. If the user's age is below 18, print a message telling the user how many years he or she has to wait before legally being allowed to drink. • Write a script that takes exactly one argument, a directory name. If the number of arguments is more or less than one, print a usage message. If the argument is not a directory, print another message. For the given directory, print the ten biggest files. (tip, you will need to use the following commands du -a, sort, head.) Introduction to Linux | Oliver Standford TOOLS & MISC Introduction to Linux | Oliver Standford grep • grep: Global Regular Expression Printer • Searches for a text pattern in files. • Prints the lines that match the pattern. grep [options] [reg expr] [filename] • grep Syntax grep oliver /etc/passwd • Searches for oliver in the system password file. Introduction to Linux | Oliver Standford Regular Expressions • A formula for matching strings that follow a pattern. • Supported by many utilities including grep, vim, sed, awk. Expression Description . Matches any single character. $ Matches the end of the line. ^ Matches the beginning of the line. * Zero or more occurrences of previous character. \ Treat the following character as an ordinary character. [] Any of the characters between the brackets. [C1 - C2] Range of the characters between C1 and C2. \<\> Matches the beginning (\<) or end (\>) of a word. \(\) Saves the characters matched by the expression into a holding area. Introduction to Linux | Oliver Standford Examples • Find 3 letter pattern with first letter r and last letter n. [oliver@ubuntu ~]$ grep r.n myfile.txt • Match lines that end with the word ‘finish’. [oliver@ubuntu ~]$ grep “finish$” myfile.txt • Match lines that begin with \begin (Latex). [oliver@ubuntu ~]$ grep “^\\begin” filename.tex Introduction to Linux | Oliver Standford Sed - Stream Editor • A pattern matching engine that can perform manipulations on lines of text. • Works on a line by line basis. • sed is a batch editor. • Syntax: • sed [options] [sed-function] [filename] [oliver@ubuntu ~]$ sed ‘s/a/b/g’ myfile.txt > newfile.txt • Change a to b in myfile.txt and direct results to newfile.txt. • s - means substitute. • g- make a global change. Introduction to Linux | Oliver Standford sed - Text Deletion • To delete use the d function. [oliver@ubuntu ~]$ sed ‘3,7d’ myfile.txt • Delete lines 3 to 7. [oliver@ubuntu ~]$ sed ‘/dogs/d’ myfile.txt • This will delete any lines that have the word ‘dogs’ in. Introduction to Linux | Oliver Standford Exercises • Use the following command to download grepdata.txt to your home directory. • wget www.lancs.ac.uk/staff/standfor/Linux/grepdata.txt • Use grep to print all lines that contain a phone number with an extension (the letter x or X followed by four digits). • Print all lines that begin with three digits followed by a blank. • Use sed to replace all ‘High School’ with ‘Secondary school’. • Delete any email addresses that are from @example.com Introduction to Linux | Oliver Standford Ssh • Linux allows remote users interactive facilities. • ssh runs an interactive terminal on a remote machine. [oli@ubuntu ~]$ ssh [email protected] • You are presented with a password prompt.
Recommended publications
  • Use Perl Regular Expressions in SAS® Shuguang Zhang, WRDS, Philadelphia, PA

    Use Perl Regular Expressions in SAS® Shuguang Zhang, WRDS, Philadelphia, PA

    NESUG 2007 Programming Beyond the Basics Use Perl Regular Expressions in SAS® Shuguang Zhang, WRDS, Philadelphia, PA ABSTRACT Regular Expression (Regexp) enhance search and replace operations on text. In SAS®, the INDEX, SCAN and SUBSTR functions along with concatenation (||) can be used for simple search and replace operations on static text. These functions lack flexibility and make searching dynamic text difficult, and involve more function calls. Regexp combines most, if not all, of these steps into one expression. This makes code less error prone, easier to maintain, clearer, and can improve performance. This paper will discuss three ways to use Perl Regular Expression in SAS: 1. Use SAS PRX functions; 2. Use Perl Regular Expression with filename statement through a PIPE such as ‘Filename fileref PIPE 'Perl programm'; 3. Use an X command such as ‘X Perl_program’; Three typical uses of regular expressions will also be discussed and example(s) will be presented for each: 1. Test for a pattern of characters within a string; 2. Replace text; 3. Extract a substring. INTRODUCTION Perl is short for “Practical Extraction and Report Language". Larry Wall Created Perl in mid-1980s when he was trying to produce some reports from a Usenet-Nes-like hierarchy of files. Perl tries to fill the gap between low-level programming and high-level programming and it is easy, nearly unlimited, and fast. A regular expression, often called a pattern in Perl, is a template that either matches or does not match a given string. That is, there are an infinite number of possible text strings.
  • A First Course to Openfoam

    A First Course to Openfoam

    Basic Shell Scripting Slides from Wei Feinstein HPC User Services LSU HPC & LON [email protected] September 2018 Outline • Introduction to Linux Shell • Shell Scripting Basics • Variables/Special Characters • Arithmetic Operations • Arrays • Beyond Basic Shell Scripting – Flow Control – Functions • Advanced Text Processing Commands (grep, sed, awk) Basic Shell Scripting 2 Linux System Architecture Basic Shell Scripting 3 Linux Shell What is a Shell ▪ An application running on top of the kernel and provides a command line interface to the system ▪ Process user’s commands, gather input from user and execute programs ▪ Types of shell with varied features o sh o csh o ksh o bash o tcsh Basic Shell Scripting 4 Shell Comparison Software sh csh ksh bash tcsh Programming language y y y y y Shell variables y y y y y Command alias n y y y y Command history n y y y y Filename autocompletion n y* y* y y Command line editing n n y* y y Job control n y y y y *: not by default http://www.cis.rit.edu/class/simg211/unixintro/Shell.html Basic Shell Scripting 5 What can you do with a shell? ▪ Check the current shell ▪ echo $SHELL ▪ List available shells on the system ▪ cat /etc/shells ▪ Change to another shell ▪ csh ▪ Date ▪ date ▪ wget: get online files ▪ wget https://ftp.gnu.org/gnu/gcc/gcc-7.1.0/gcc-7.1.0.tar.gz ▪ Compile and run applications ▪ gcc hello.c –o hello ▪ ./hello ▪ What we need to learn today? o Automation of an entire script of commands! o Use the shell script to run jobs – Write job scripts Basic Shell Scripting 6 Shell Scripting ▪ Script: a program written for a software environment to automate execution of tasks ▪ A series of shell commands put together in a file ▪ When the script is executed, those commands will be executed one line at a time automatically ▪ Shell script is interpreted, not compiled.
  • Shell Scripting, Scripting Examples

    Shell Scripting, Scripting Examples

    Last Time… on the website Lecture 6 Shell Scripting What is a shell? • The user interface to the operating system • Functionality: – Execute other programs – Manage files – Manage processes • Full programming language • A program like any other – This is why there are so many shells Shell History • There are many choices for shells • Shell features evolved as UNIX grew Most Commonly Used Shells – /bin/csh C shell – /bin/tcsh Enhanced C Shell – /bin/sh The Bourne Shell / POSIX shell – /bin/ksh Korn shell – /bin/bash Korn shell clone, from GNU Ways to use the shell • Interactively – When you log in, you interactively use the shell • Scripting – A set of shell commands that constitute an executable program Review: UNIX Programs • Means of input: – Program arguments [control information] – Environment variables [state information] – Standard input [data] • Means of output: – Return status code [control information] – Standard out [data] – Standard error [error messages] Shell Scripts • A shell script is a regular text file that contains shell or UNIX commands – Before running it, it must have execute permission: • chmod +x filename • A script can be invoked as: – ksh name [ arg … ] – ksh < name [ args … ] – name [ arg …] Shell Scripts • When a script is run, the kernel determines which shell it is written for by examining the first line of the script – If 1st line starts with #!pathname-of-shell, then it invokes pathname and sends the script as an argument to be interpreted – If #! is not specified, the current shell assumes it is a script in its own language • leads to problems Simple Example #!/bin/sh echo Hello World Scripting vs.
  • Unix (And Linux)

    Unix (And Linux)

    AWK....................................................................................................................................4 BC .....................................................................................................................................11 CHGRP .............................................................................................................................16 CHMOD.............................................................................................................................19 CHOWN ............................................................................................................................26 CP .....................................................................................................................................29 CRON................................................................................................................................34 CSH...................................................................................................................................36 CUT...................................................................................................................................71 DATE ................................................................................................................................75 DF .....................................................................................................................................79 DIFF ..................................................................................................................................84
  • Chapter 10 SHELL Substitution and I/O Operations

    Chapter 10 SHELL Substitution and I/O Operations

    Chapter 10 SHELL Substitution and I/O Operations 10.1 Command Substitution Command substitution is the mechanism by which the shell performs a given set of commands and then substitutes their output in the place of the commands. Syntax: The command substitution is performed when a command is given as: `command` When performing command substitution make sure that you are using the backquote, not the single quote character. Example: Command substitution is generally used to assign the output of a command to a variable. Each of the following examples demonstrate command substitution: #!/bin/bash DATE=`date` echo "Date is $DATE" USERS=`who | wc -l` echo "Logged in user are $USERS" UP=`date ; uptime` echo "Uptime is $UP" This will produce following result: Date is Thu Jul 2 03:59:57 MST 2009 Logged in user are 1 Uptime is Thu Jul 2 03:59:57 MST 2009 03:59:57 up 20 days, 14:03, 1 user, load avg: 0.13, 0.07, 0.15 10.2 Shell Input/Output Redirections Most Unix system commands take input from your terminal and send the resulting output back to your terminal. A command normally reads its input from a place called standard input, which happens to be your terminal by default. Similarly, a command normally writes its output to standard output, which is also your terminal by default. Output Redirection: The output from a command normally intended for standard output can be easily diverted to a file instead. This capability is known as output redirection: If the notation > file is appended to any command that normally writes its output to standard output, the output of that command will be written to file instead of your terminal: Check following who command which would redirect complete output of the command in users file.
  • Bash Guide for Beginners

    Bash Guide for Beginners

    Bash Guide for Beginners Machtelt Garrels Garrels BVBA <tille wants no spam _at_ garrels dot be> Version 1.11 Last updated 20081227 Edition Bash Guide for Beginners Table of Contents Introduction.........................................................................................................................................................1 1. Why this guide?...................................................................................................................................1 2. Who should read this book?.................................................................................................................1 3. New versions, translations and availability.........................................................................................2 4. Revision History..................................................................................................................................2 5. Contributions.......................................................................................................................................3 6. Feedback..............................................................................................................................................3 7. Copyright information.........................................................................................................................3 8. What do you need?...............................................................................................................................4 9. Conventions used in this
  • Introduction to Perl

    Introduction to Perl

    Introduction to Perl Science and Technology Support Group High Performance Computing Ohio Supercomputer Center 1224 Kinnear Road Columbus, OH 43212-1163 Introduction to Perl • Setting the Stage • Functions • Data Types • Exercises 3 • Operators • File and Directory Manipulation • Exercises 1 • External Processes • Control Structures • References • Basic I/O • Exercises 4 • Exercises 2 • Some Other Topics of Interest • Regular Expressions • For Further Information 2 Introduction to Perl Setting the Stage • What is Perl? • How to get Perl • Basic Concepts 3 Introduction to Perl What is Perl? • Practical Extraction and Report Language –Or: Pathologically Eclectic Rubbish Lister • Created by Larry Wall • A compiled/interpreted programming language • Combines popular features of the shell, sed, awk and C • Useful for manipulating files, text and processes – Also for sysadmins and CGI • Latest official version is 5.005 – Perl 4.036 still in widespread use –Use perl -v to see the version • Perl is portable •Perlis free! 4 Introduction to Perl How to Get Perl • Perl’s most natural “habitat” is UNIX • Has been ported to most other systems as well – MS Windows, Macintosh, VMS, OS/2, Amiga… • Available for free under the GNU Public License – Basically says you can only distribute binaries of Perl if you also make the source code available, including the source to any modifications you may have made • Can download source code (C) and compile yourself (unlikely to be necessary) • Pre-compiled binaries also available for most systems • Support available
  • A Crash Course on UNIX

    A Crash Course on UNIX

    AA CCrraasshh CCoouurrssee oonn UUNNIIXX UNIX is an "operating system". Interface between user and data stored on computer. A Windows-style interface is not required. Many flavors of UNIX (and windows interfaces). Solaris, Mandrake, RedHat (fvwm, Gnome, KDE), ... Most UNIX users use "shells" (or "xterms"). UNIX windows systems do provide some Microsoft Windows functionality. TThhee SShheellll A shell is a command-line interface to UNIX. Also many flavors, e.g. sh, bash, csh, tcsh. The shell provides commands and functionality beyond the basic UNIX tools. E.g., wildcards, shell variables, loop control, etc. For this tutorial, examples use tcsh in RedHat Linux running Gnome. Differences are minor for the most part... BBaassiicc CCoommmmaannddss You need these to survive: ls, cd, cp, mkdir, mv. Typically these are UNIX (not shell) commands. They are actually programs that someone has written. Most commands such as these accept (or require) "arguments". E.g. ls -a [show all files, incl. "dot files"] mkdir ASTR688 [create a directory] cp myfile backup [copy a file] See the handout for a list of more commands. AA WWoorrdd AAbboouutt DDiirreeccttoorriieess Use cd to change directories. By default you start in your home directory. E.g. /home/dcr Handy abbreviations: Home directory: ~ Someone else's home directory: ~user Current directory: . Parent directory: .. SShhoorrttccuuttss To return to your home directory: cd To return to the previous directory: cd - In tcsh, with filename completion (on by default): Press TAB to complete filenames as you type. Press Ctrl-D to print a list of filenames matching what you have typed so far. Completion works with commands and variables too! Use ↑, ↓, Ctrl-A, & Ctrl-E to edit previous lines.
  • Minimal Perl for UNIX and Linux People

    Minimal Perl for UNIX and Linux People

    Minimal Perl For UNIX and Linux People BY TIM MAHER MANNING Greenwich (74° w. long.) For online information and ordering of this and other Manning books, please visit www.manning.com. The publisher offers discounts on this book when ordered in quantity. For more information, please contact: Special Sales Department Manning Publications Co. Cherokee Station PO Box 20386 Fax: (609) 877-8256 New York, NY 10021 email: [email protected] ©2007 by Manning Publications Co. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by means electronic, mechanical, photocopying, or otherwise, without prior written permission of the publisher. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in the book, and Manning Publications was aware of a trademark claim, the designations have been printed in initial caps or all caps. Recognizing the importance of preserving what has been written, it is Manning’s policy to have the books we publish printed on acid-free paper, and we exert our best efforts to that end. Manning Publications Co. Copyeditor: Tiffany Taylor 209 Bruce Park Avenue Typesetters: Denis Dalinnik, Dottie Marsico Greenwich, CT 06830 Cover designer: Leslie Haimes ISBN 1-932394-50-8 Printed in the United States of America 12345678910–VHG–1009080706 To Yeshe Dolma Sherpa, whose fortitude, endurance, and many sacrifices made this book possible. To my parents, Gloria Grady Washington and William N. Maher, who indulged my early interests in literature. To my limbic system, with gratitude for all the good times we’ve had together.
  • Shells and Shell Programming

    Shells and Shell Programming

    Shells & Shell Programming (Part A) Software Tools EECS2031 Winter 2018 Manos Papagelis Thanks to Karen Reid and Alan J Rosenthal for material in these slides SHELLS 2 What is a Shell • A shell is a command line interpreter that is the interface between the user and the OS. • The shell: – analyzes each command – determines what actions are to be performed – performs the actions • Example: wc –l file1 > file2 3 Which shell? • sh – Bourne shell – Most common, other shells are a superset – Good for programming • csh or tcsh – command-line default on EECS labs – C-like syntax – Best for interactive use. • bash – default on Linux (Bourne again shell) – Based on sh, with some csh features. • korn – written by David Korn – Based on sh – Some claim best for programming. – Commercial product. 4 bash versus sh • On EECS labs, when you run sh, you are actually running bash. • bash is a superset of sh. • For EECS2031, you will be learning only the features of the language that belong to sh. 5 Changing your shell • I recommend changing your working shell on EECS to bash – It will make it easier to test your shell programs. – You will only need to learn one set of syntax. • What to do: – echo $SHELL (to check your current shell) – chsh <userid> bash – Logout and log back in. – .profile is executed every time you log in, so put your environment variables there 6 Standard Streams • Preconnected input and output channels between a computer program and its environment. There are 3 I/O connections: – standard input (stdin) – standard output (stdout) – standard
  • Linux Programming [R15a0527] Lecture Notes

    Linux Programming [R15a0527] Lecture Notes

    LINUX PROGRAMMING [R15A0527] LECTURE NOTES B.TECH IV YEAR – I SEM (R15) (2019-2020) DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING MALLA REDDY COLLEGE OF ENGINEERING & TECHNOLOGY (Autonomous Institution – UGC, Govt. of India) Recognized under 2(f) and 12 (B) of UGC ACT 1956 (Affiliated to JNTUH, Hyderabad, Approved by AICTE - Accredited by NBA & NAAC – ‘A’ Grade - ISO 9001:2015 Certified) Maisammaguda, Dhulapally (Post Via. Hakimpet), Secunderabad – 500100, Telangana State, India Syllabus (R15A0527) LINUX PROGRAMMING Objectives: • To develop the skills necessary for Unix systems programming including file system programming, process and signal management, and interprocess communication. • To make effective use of Unix utilities and Shell scripting language such as bash. • To develop the basic skills required to write network programs using Sockets. UNIT I Linux Utilities - File handling utilities, Security by file permissions, Process utilities, Disk utilities, Networking commands, Filters, Text processing utilities and Backup utilities. Sed- Scripts, Operation, Addresses, Commands, Applications, awk- Execution, Fields and Records, Scripts, Operation, Patterns, Actions, Associative Arrays, String and Mathematical functions, System commands in awk, Applications. Shell programming with Bourne again shell(bash) - Introduction, shell responsibilities, pipes and Redirection, here documents, running a shell script, the shell as a programming language, shell meta characters, file name substitution, shell variables, command substitution, shell
  • Unix Login Profile

    Unix Login Profile

    Unix login Profile A general discussion of shell processes, shell scripts, shell functions and aliases is a natural lead in for examining the characteristics of the login profile. The term “shell” is used to describe the command interpreter that a user runs to interact with the Unix operating system. When you login, a shell process is initiated for you, called your login shell. There are a number of "standard" command interpreters available on most Unix systems. On the UNF system, the default command interpreter is the Korn shell which is determined by the user’s entry in the /etc/passwd file. From within the login environment, the user can run Unix commands, which are just predefined processes, most of which are within the system directory named /usr/bin. A shell script is just a file of commands, normally executed at startup for a shell process that was spawned to run the script. The contents of this file can just be ordinary commands as would be entered at the command prompt, but all standard command interpreters also support a scripting language to provide control flow and other capabilities analogous to those of high level languages. A shell function is like a shell script in its use of commands and the scripting language, but it is maintained in the active shell, rather than in a file. The typically used definition syntax is: <function-name> () { <commands> } It is important to remember that a shell function only applies within the shell in which it is defined (not its children). Functions are usually defined within a shell script, but may also be entered directly at the command prompt.