Unix Shells: C, Bourne, Bourne Again, Korn, and Tenex Shells Presentation by: Katie Harris Andrew Murray

Introduction Numerous types of shells offered Why so many shells? Some are original shells and others combine features of the original shells 5 shells: Bourne Shell, Bourne Again Shell, C Shell, Korn Shell, and Tenex Shell

Bourne Shell Default Unix Shell created in 1977 by Stephen Bourne The UNIX Programming Environment Foundation for future shells Very strong and powerful syntactical language Controlling input and output – File Descriptor 2 Larger argument list size and the idea of having environment variables

C Shell Developed by Bill Joy at the University of California at Berkeley Extensive command language with syntax similar to the C programming language Job control features of the BSD kernel Never very popular Aliases Command History Bourne = Programming, C-Shell = Command Interpretation

Bash Shell Bourne-again shell developed as a part of the GNU project by Brian Fox in 1987 Default shell on most GNU/Linux and Mac OS X Panther systems Freely distributed and combines C-Shell and Bourne Shell File completion Easier History Use

Korn Shell Created by David Korn in AT&T Labs in 1983 Compatible with Bourne Shell Utilizes both features from the Bourne Shell and the C Shell Some features include: coprocesses and one- dimensional arrays The Korn shell is an interactive command interpreter and command programming language. The shell carries out commands specified at the terminal or from a file.

Tenex Shell Created by Ken Greer Tcsh is an enhanced, but completely compatible version of the Berkeley UNIX C shell (csh). It is a command language interpreter usable both as an interactive login shell and a shell script command processor. It includes a command-line editor, programmable word completion, spelling correction, a history mechanism, job control and a C-like syntax.

Shell Features sh csh ksh bash tcsh sh csh ksh bash tcsh Job control N Y Y Y Y Aliases N Y Y Y Y Aliases N Y Y Y Y Shell functions Y(1) N Y Y N "Sensible" Input/Output redirectionY N Y Y N "Sensible" Input/Output redirectionY N Y Y N Directory stack N Y Y Y Y Command history N Y Y Y Y Command line editing N N Y Y Y Vi Command line editing N N Y Y Y(3) Emacs Command line editing N N Y Y Y Rebindable Command line editing N N N Y Y User name look up N Y Y Y Y

Shell Features(2) sh csh ksh bash tcsh sh csh ksh bash tcsh Login/Logout watching N N N N Y Filename completion N Y(1) Y Y Y Username completion N Y(2) Y Y Y Hostname completion N Y(2) Y Y Y History completion N N N Y Y Fully programmable Completion N N N N Y Mh Mailbox completion N N N N(4) N(6) Co Processes N N Y N N Builtin artithmetic evaluation N Y Y Y Y Can follow symbolic links invisibly N Y Y Y Y Periodic command execution N N N N Y Custom Prompt (easily) N N Y Y Y Sun Keyboard Hack N N N N N Spelling Correction N N N N Y

Shell Features(3) sh csh ksh bash tcsh sh csh ksh bash tcsh Process Substitution N N N Y(2) N Underlying Syntax sh csh sh sh csh Freely Available N N N(5) Y Y Checks Mailbox N Y Y Y Y Tty Sanity Checking N N N N Y Can cope with large argument lists Y N Y Y Y Has non-interactive startup file N Y Y(7) Y(7) Y Has non-login startup file N Y Y(7) Y Y Can avoid user startup files N Y N Y N Can specify startup file N N Y Y N Low level command redefinition N N N N N Has anonymous functions N N N N N List Variables N Y Y N Y Full signal trap handling Y N Y Y N

Shell Features(4) sh csh ksh bash tcsh sh csh ksh bash tcsh File no clobber ability N Y Y Y Y Local variables N N Y Y N Lexically scoped variables N N N N N Exceptions N N N N N Y Feature can be done using this shell. N Feature is not present in the shell. F Feature can only be done by using the shells function mechanism. L The readline library must be linked into the shell to enable this Feature. 1.This feature was not in the orginal version, but has since become almost standard. 2.This feature is fairly new and so is often not found on many versions of the shell, it is gradually making its way into standard distribution. 3.The Vi emulation of this shell is thought by many to be incomplete. 4.This feature is not standard but unoffical patches exist to perform this. 5.A version called 'pdksh' is freely available, but does not have the full functionality of the AT&T version. 6.This can be done via the shells programmable completion mechanism. 7.Only by specifing a file via the ENV environment variable.

Redirection and Pipes

Common Features to Bourne, Korn, and C Shells Symbol/CommandMeaning/Action >Redirect output >>Append to file <Redirect input <<“Here” document (redirect input) |Pipe output |&Start a coprocess (Korn shell only) &Run process in background ;Separate commands on same line *Match any character(s) in filename ?Match single character in filename [ ]Match any characters enclosed ( )Execute in subshell ` `Substitute output of enclosed command “ “Partial quote(allows var. and command expansion ‘ ‘Full quote(no expansion)

Common Features to Bourne, Korn, and C Shells(2) Symbol/CommandMeaning/Action \Quote following character $varUse value for variable $$Process ID $0Command name $nnth argument (0<=n<=9) $*All arguments as simple words #Begin comment bgBackground execution breakBreak from loop statements cdChange directory continueResume a program loop echoDisplay output evalEvaluate arguments execExecute a new shell fgForeground execution

Common Features to Bourne, Korn, and C Shells(3) Symbol/CommandMeaning/Action jobsShow active jobs killTerminate running jobs shiftShift positional parameters stopSuspend a background job suspendSuspend a foreground job timeTime a command umaskSet default file permissions for new files unsetErase variable or function definitions waitWait for a background job to finish

Differing Features of Bourne, Korn, and C Shells shkshcshMeaning/Action $$%Prompt >|>!Force redirection >file 2>&1>file 2>&1>& fileCombine stdout and stderr { }Expand elements in list ` `` `` `Substitute output of enclosed command $( )Substitute output of enclosed command $HOME$HOME$homeHome directory ~~Home directory symbol var=valuevar=valueset var=valueVariable assignment export varexport var=valsetenv var valSet environment variable ${nn}More than nine args can be referenced args as separate words $#$#$#argvNumber of arguments

Differing Features of Bourne, Korn, and C Shells(2) shkshcshMeaning/Action $?$?$statusExit status $!$!Background exit status $-$-Current options. file. filesource fileRead commands in file alias x=yalias x yName x stands for y casecaseswitch/caseChoose alternatives cd ~-popd/pushdSwitch directories donedoneendEnd a loop statement esacesacendswEnd case or switch exit [n]exit [n]exit [(expr)]Exit with a status for/dofor/doforeachLoop through variables print –rglobIgnore echo escapes hashalias –thashstatDisplay hashed comments hash cmdsalias –t cmdsrehashRemember command locations hash –rPATH=$PATHunhashForget command locations historyhistoryList previous commands

Differing Features of Bourne, Korn, and C Shells(3) shkshcshMeaning/Action r!!Redo previous command r str!strRedo command that starts with str\ r x=cmd!cmd:s/x/yEdit command, then execute if [ $i –eq 5]if ((i==5))if ($i==5)Sample if statement fifiendifEnd if statement ulimitulimitlimitSet resource limits pwdpwddirsPrint working directory readread$<Read from standard input trap 2trap 2onintrIgnore interrupts unaliasunaliasRemove aliases until/dountil/doBegin until loop while/dowhile/dowhileBegin while loop

Shell control structures StructureBourneKornC Shell IFif[…]if[…]if(…) then THENthenthen ELSE-IFelifelifElse if ELSEelseelseelse ENDIFfifiendif CASEcasecaseswitch value) value) case value: value) value) case value: ;; ;; breaksw ;; ;; breaksw * ) * ) default: * ) * ) default: esacesacendsw FORforforforeach Dodo donedoneend REPEATxargs -1xargs -1xargs -1 repeat UNTILuntiluntil Dodo donedone WHILEwhilewhilewhile Dodo donedoneend

Conclusion Numerous shells but all essential to the evolution of the Unix system and the shell Build off of each other Main building block for all shells in one form or another is the Bourne Shell

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