Chapter 3 The UNIX Shells

Introduction What is shell ? What kinds of shell are available in UNIX command interpreter an interface between a user and the raw os What kinds of shell are available in UNIX Bourne shell Korn shell C shell Korn shell C shell Bourne shell core core

Selecting a Shell Shell functionality Selecting a shell built-in commands, scripts, variables, redirection wildcards, pipes, sequences subshells, background processing, command substitution Selecting a shell The system administrator chooses a shell for you. /etc/passwd Full pathname of Shells Bourne /bin/sh , /bin/bash Korn /bin/ksh C shell /bin/csh

Selecting a Shell How to change your shell ? %chsh Changing login shell for chang Old shell : /bin/csh New shell : /bin/ksh %logout login : chang passwd: $

Shell sequence read a startup file in the user's home directory display a prompt and waits for a user command execute the user's command Control-D Terminate

Startup Files Bourne shell : /bin/sh Bourne again shell : /bin/bash .profile .login Bourne again shell : /bin/bash .bashrc C shell : /bin/csh .cshrc

Executable files vs. built-in commands several built-in commands in Shell % echo -n {arg}* % cd directory utility programs (a file with execute permission) stored in a directory Shell locates and executes utilities usually in shell variable $PATH /bin, /usr/bin, /usr/local/bin, /usr/ucb, /etc, /usr/bin/X11 % ls ... /bin/ls

Output Redirection store the output to a file append %command >fileName %ls > sample.txt append %command>>fileName %cat >> sample.txt

Input Redirection Input redirection Here documents execute command using the file as its standard input %command < fileName %elm chang < sample.txt Here documents %command << word … word

Pipe Pipes the standard output of command1 as the standard input of command2 %command1 |command2 %ls | wc -w

Command substitution %echo the date today is `date` A command surrounded by backquote(`) is replaced by its standard output. %echo the date today is `date`

Combination of commands Grouping %(command1; ... ; commandn) executed by a child shell (subshell) shares the same standard input, standard output, and standard error redirected and piped as a single command %date; ls; pwd > out.txt %(date; ls; pwd) > out.txt

Combination of commands Sequences %command1; …; commandn the shell executes them in sequence, from left to right Conditional sequence % command1 && command2 command2 is executed only if command1 completes successfully (return 0) %cc my prog.c && a.out % command1 || command2 command2 is executed only if command1 complete unsuccessfully %cc myprog.c || echo compilation failed

Filename substitution specify several files using wildcards Example %ls *.c %ls */* %ls [ac]* %cc *.c Wildcard Meaning * Matches any string, including the empty string ? Matches any single character [..] Matches any one of the characters between the brackets. A range of characters may be specified.

Background processing Examples % find . -name b.c -print & 27174 %date & pwd & 27310 27311 %command & command shell

Background processing Redirecting background processes To prevent the output from a background process from arriving at your terminal redirect output to a file %find .-name a.c -print > find.txt & mail output to your self % find . -name a.c -print | mail glass & redirecting input % mail glass < inputfile &

Shell Script What is Shell script ? Which shell script 1.A file that contains a sequence of shell commands 2. Need execute permission. 3. Run it by just typing its name Which shell script 1. C shell script, if the first line is # 2. If the first line is #!pathName, the executable program pathName is used to interpret the script. 3. Otherwise, it is Bourne shell script.

Shell Script: Example script.csh #This is sample C shell scripts echo -n the date today is date script.ksh #!/bin/ksh #This is sample Korn shell scipt. 실행 준비 %chmod 700 script.csh script.ksh

Subshell Parent  An initial login shell when you log into UNIX Parent  When your current shell(parent) creates a new (child) shell ? 1. When a grouped command is executed, such as (ls; pwd; date). 2. When a script is executed. 3. When a background job is executed. Parent Child Parent Env. vars Local vars Child Env. vars Local vars

Environment variables vs Local variables  Every shell contains two data areas 1. environment variable space A child shell inherits a copy of its parent's 2. local variable space A child shell get a clean local variable space

Environment variables Predefined environment variables %echo HOME=$HOME, PATH=$PATH, MAIL=$MAIL

Local variables  Local variables - shell variables except predefined environment variables - export make local variables environment variables  local built-in variables

Example $firstname=Graham $lastname=Glass $echo $firstname $lastname $export lastname $sh $^D

Quoting  To inhibit the shell's wildcard replacement, variable substitution, and command substitution 1. single quote (') inhibit wildcard replacement, variable substitution, and command substitution 2. double quote (") inhibit wildcard replacement only 3. when quotes are nested, the outer quote have any effect.

Quoting  Examples % echo 3 * 4 = 12 % echo "3 * 4 = 12" % set name = Graham % echo 'my name is $name and the date is 'date`' % echo "my name is $name and the date is 'date' "

Job Control  Process status: ps ps -arux list process status information, by default your processes. -a option : all processes  sleep seconds sleeps for the specified number of seconds and then terminate  wait [pid] The shell suspends until the child process with pid terminates. If no arguments are supplied, the shell waits for all its child processes

Job Control  Signalling process : kill kill [-signalId] {pid}+ kill -l 1. Kill(TERM signal) process or 2. sends the signal with code signalId to the list of numbered processes. %(sleep 10; echo done) & %ps 27390 %kill -9 27390 %sleep 30 & sleep30 & sleep 30 %kill –9 0