Shell Games

• Objective: To introduce students to the concept of a shell, effective use of the shell, the shell as programming language, and shell scripts. We will use the bash shell for the examples and details provided.– Shell syntax and commands – history and command completion– job control– variables– programming constructs– scripts

shell

• command interpreter (bash, sh, csh,…)• .bashrc, .profile• PATH and shell variables• metacharacters• history and command completion• file redirection• pipes• process management

What does the shell do?

• In Unix, separate from the OS (change look and feel)

• reads and executes commands– some handled by the shell itself (pwd, echo,…)– some are programs stored in some directory (look in

directories in PATH). Start a subshell to execute these

• Provides support for better interaction with the computer/OS (command history, editing, configuration)

• Programming language

Executing a Command

• After reading a command, the shell may do some processing (see wildcards etc in the syntax description that follows), then it must find a program to execute the command.

• Some commands are executed directly by the shell. Other commands are executed by separate programs. These are found by looking in a list of directories for programs with the appropriate name. The shell searches directories in the PATH variable. A hash table is used to make the search fast. You can add new commands simply by adding new programs (a program can be any executable file including scripts – review Unix permissions) to directories in the PATH. You can modify/add directories in the PATH.

Finding out about Commands

• type echo

• which echo

• info echo

• man echo

• info bash

Example[jjohnson@ws44 cs265]$ which echo/bin/echo[jjohnson@ws44 cs265]$ type -a echoecho is a shell builtinecho is /bin/echo[jjohnson@ws44 cs265]$ help echoecho: echo [-neE] [arg ...] Output the ARGs. If -n is specified, the trailing newline is suppressed. If the -e option is given, interpretation of the following backslash-escaped characters is turned on: \a alert (bell) \b backspace \c suppress trailing newline \E escape character \f form feed \n new line \r carriage return \t horizontal tab \v vertical tab \\ backslash \num the character whose ASCII code is NUM (octal). You can explicitly turn off the interpretation of the above characters with the -E option.

Notes about PATH

• If you do not have . in your PATH, commands in your current directory will not be found. You may or may not want to add . to your PATH. If you do not and want to execute a command in the current directory– ./command

• The PATH is searched sequentially, the first matching program is executed. Beware of naming your executables test as there is another program called test and this may be executed when you enter– test

PATH[jjohnson@ws44 software]$ echo $PATH

/usr/local/FrameMaker/bin:/home/jjohnson/bin:/usr/local/gpl/mpich-1.2.4/bin:/usr/local/bin:/usr/sbin:/sbin:/usr/openwin/bin:/opt/SUNWspro/bin:/usr/ccs/bin:/usr/ucb:/usr/sbin:/usr/bin:/etc:/usr/etc:/usr/UTILS/publisher/bin:/usr/bin/X11:/bin:/usr/remote/alg_soft/linda2.5.2sol2.3/bin:.

[jjohnson@ws44 software]$ PATH=${PATH}:/home/jjohnson/software

[jjohnson@ws44 software]$ echo $PATH

/usr/local/FrameMaker/bin:/home/jjohnson/bin:/usr/local/gpl/mpich-1.2.4/bin:/usr/local/bin:/usr/sbin:/sbin:/usr/openwin/bin:/opt/SUNWspro/bin:/usr/ccs/bin:/usr/ucb:/usr/sbin:/usr/bin:/etc:/usr/etc:/usr/UTILS/publisher/bin:/usr/bin/X11:/bin:/usr/remote/alg_soft/linda2.5.2sol2.3/bin:.:/home/jjohnson/software

Aliases

• Commands can be aliased (renamed) to alter their behavior. A common used of aliases is to add options to the default behavior of certain commands (e.g. ls, rm, mv, …)

• It is common practice to alias rm to prompt the user to make sure that the specified files should be removed. This is especially important since you can remove all files with (see wildcards)– rm *

• To see current aliases, use the alias command, this can also be used to set new aliases.

Aliases[jjohnson@ws44 jjohnson]$ aliasalias cd..='cd ..'alias cp='cp -i'alias d='ls'alias df='df -h -x supermount'alias du='du -h'alias kde='xinit /usr/bin/startkde'alias l='ls'alias la='ls -a'alias ll='ls -l'alias ls='ls -F --color=auto'alias lsd='ls -d */'alias md='mkdir'alias mv='mv -i'alias p='cd -'alias rd='rmdir'alias rm='rm -i'alias s='cd ..'[jjohnson@ws44 jjohnson]$ type rmrm is aliased to `rm -i'

Shell Syntax

• Comments– # This is a comment– ls # list the files in the current directory

• Line continuation– echo A long \– > line

• ; #Command separator – you can list more than one command per line separated by ;– ls ; who

• \ #Pathname separator– cd \home\jjohnson

Shell Syntax

• Wildcards, and pathname expansion– * # match any string – ? # match any single character– [set] # match characters listed in set (can be range)– [!set] # mach any character not given in set

• Examples– ls *.c – ls *.?– ls *.[Hh][Tt][Ll]– ls [a-z]

Shell Syntax

• File redirection and pipes– < # redirect input from specified source– > # redirect output to specified source– >> # redirect output and append to specified source– | # pipe the output from one command to the input to the

next

• Examples– grep word < /usr/dict/words– ls > listing– ls >> listing– ls -l| wc -l

Shell Syntax

• Note file redirection of standard output [stdout] does not include error messages, which go to standard error [stderr] (when you are in a terminal session, both stdout and stderr go to the screen; however, when you redirect stdout to a file, stderr still goes to the screen).

• stdout is designated by the file descriptor 1 and stderr by 2 (standard input is 0)– To redirect standard error use 2>– ls filenothere > listing 2> error– ls filenothere 2>&1 > listing # both stdout and stderr

redirected to listing

Shell Syntax

• Background jobs– & # run command in the background– grep ‘we.*’< /usr/word/dict > wewords &– This runs the grep command in the background – you

immediately get a new prompt and can continue your work while the command is run. Note that the output was redirected otherwise you would lose it.

– To find out the jobs that are running in the back use the command jobs (also see the command ps)

– To stop a job use the kill command (you need to know the process id of the job [use –l option to jobs or ps]

Examplejjohnson@HILBERT ~$ find / -name me -print &[1] 1936

jjohnson@HILBERT ~$ jobs -l[1]+ 1936 Running find / -name me -print &

jjohnson@HILBERT ~$ ps PID PPID PGID WINPID TTY UID STIME COMMAND 1608 1 1608 1608 con 1000 21:52:42 /usr/bin/bash 1936 1608 1936 1816 con 1000 09:18:03 /usr/bin/find 2036 1608 2036 1488 con 1000 09:18:07 /usr/bin/ps

jjohnson@HILBERT ~$ kill 1936

jjohnson@HILBERT ~$ jobs[1]+ Terminated find / -name me -print

Shell Syntax

• Control characters (hold the Ctrl key and the specified key at the same time)– CTRL-C # interupt – default behavior is to stop the

current command – this is very handy for infinite loops, pages of output, etc.

– CTRL-\ # quite – use when CTRL-C does not stop the current command

– CTRL-Z # suspend the current command, you can continue the command either in the foreground with fg or the background with bg

– CTRL-D # end of input (useful when entering input from standard input, stdin)

quoting• Sometimes you do not want the shell to interpret

metacharacters such as * or > when entering a command. Use single quotes to prevent the shell from evaluating an expression.– echo 2 * 3 > 5 is a valid inequality– echo ‘2 * 3 > 5 is a valid inequality’– echo ‘2*3 > 5’ is a valid inequality

• There are two types of single quotes (forward and back – the previous quote was a forward quote). back quote has a different meaning – it causes the shell to evaluate the expression in the back quotes.– echo `2 * 3 > 5` is a valid inequality

quoting

• Alternatively, you can turn of the interpretation of special characters one at a time with backslash-escaping– echo 2 \* 3 \> 5 is a valid inequality

variables

• Arbitrary variables can be used in the shell, assign with =, and obtain value with $.– name=jeremy– echo $name

• Sometimes it is useful to include the value of a variable as part of another expression.– echo ${name}.file

Environment Variables

• Variables defined are not passed, by default, to subshells (recall that every time a command is executed another shell is created to run the command). To export variables so that they are passed to subshells use the export command.– NAME=jeremy– export $NAME

Environment Variables

• Environment variables are typically used to configure your environment and to store useful information about your environment. Some of these are defined in the system profile file. You can define additional ones in your .profile file.

• Some useful Environment variables – USER, SHELL, HOST, PATH, PS1, PWD

• Use the command printenv to see the currently defined environment variables

strong vs. weak quoting

• Single quote turns off all interpretation, sometimes you want partial evaluation. Weak quoting, using double quotes, does partial evaluation. In particular, variables are still evaluated

• Example– echo ‘User = $USER’

– echo “User = $USER”

Simple Script Version 1

$ who | wc –l

Put this in a file called nu (make sure permission is set to execute)

$ cat nu

#!/bin/bash

# Return the number of users logged on

who | wc -l

Special Variables

• $# the number of arguments• $* all arguments• $@ all arguments• $? return value of last command executed• $$ process id of shell• $HOME, $IFS, $PATH, $PS1, $PS2• $(pwd)• $((expr))

Exit Status

• Every UNIX command, whether it comes from source code in C or some other language, a script, or builtin returns an integer code to its calling process.

• Use exit to return value

• To obtain the returned value use $?

Example$ lsfile1 file10 file2 file3 file4 file5 file6 file7 file8 file9

jjohnson@HILBERT ~/cs265$ cat file1stuff

jjohnson@HILBERT ~/cs265$ echo $?0

jjohnson@HILBERT ~/cs265$ cat file0cat: file0: No such file or directory

jjohnson@HILBERT ~/cs265$ echo $?1

Conditional

# [ ] evaluates exit status – note must have surrounding spaces

if [ -z “$1” ]; then

echo ‘no arguments’

else

echo $1

fi

Case Statement

case $opt in

a ) echo "option a";;

b ) echo "option b";;

c ) echo "option c";;

\? ) echo 'usage: alice [-a] [-b] [-c] args...'

exit 1;;

esac

Loops

for file in *

do

echo $file

done

for file in *; do echo $file; done

Loops

i=0

while [ $i –lt 10 ]

do

echo $i

i=$((i+1))

done

Script Version 2 (Parameters)

#!/bin/bash

# Check to see if specified user is logged on

who | grep $1

Script Version 3 (conditional)

#!/bin/bash

# Check to see if specified user is logged on

if [ -z “$1” ]; then

echo “usage: nu username”

exit 1

fi

who | grep $1

Script Version 3 (for loop)

#!/bin/bash

# Check to see if specified user is logged on

if [ -z “$1” ]; then

echo “usage: nu (username)+”

exit 1

fi

for user $*; do

who | grep $user

done

Script Version 4 (while loop)

#!/bin/bash

# Check to see if specified user is logged on

if [ -z “$1” ]; then

echo “usage: nu (username)+”

exit 1

fi

while [ ! –z $1 ]; do

who | grep $1

shift

done