Shell Script Programming 2 - Santa Monica College

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© David Morgan 2011-13

Shell Script Programming 2Shell Script Programming 2

David Morgan


Useful capabilitiesUseful capabilities� parameter processing

� validation– usage checking

– user input

� custom functions

� filenames – decomposition, tempnames, random names

� action hinged on compound condition evaluation (ifless if)

� sourcing– execute commands from a file in current shell

– “#include” functions in such a file, by reference

� debugging

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Supplemental supporting utilitiesSupplemental supporting utilities

� find – generate qualified file name lists

� xargs – build command lines from stdin

� sed – dynamic, programatic, in-place editing

� awk – text manipulation

� bc – arbitrary precision math

� capabilities lacking or weak in shell

� often utilized in scripts


Special shell variablesSpecial shell variables

� $0 – script name from command line

� $1, $2, etc – command line parameters

� $* - command line parameters collectively

� $$ - process ID (PID) of current process

� $? – exit status of last command

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shift shift –– positional parameter promoterpositional parameter promoter


getoptsgetopts

� getopts vs getopt

– a shell built-in

– a stand-alone binary

– not identical

� processes command-line options

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CommandCommand--line formatline format

command [ - options ] [ arguments ]

ls -l m*

tokens


Some options take a value,Some options take a value,

some commands take argumentssome commands take arguments

myprog [ -x value ] [-y] [ aaa ] [ bbb ] [ ccc ]

command [ - options ] [ arguments ]

an option (-d ) taking a value ( : )

a command ( cut ) taking an argument ( /etc/passwd )

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““getoptsgetopts”” commandcommand

� resides in a script

� knows the script’s allowable options

� parses the command line that called the script

� executed successively, finds/returns the options in the line one at a time

� used in loops to read/detect them one after the other

� detect phase normally followed by programmed response phase


““getoptsgetopts”” documentationdocumentation (from (from ““man bashman bash””))

getopts optstring name [args]

getopts is used by shell procedures to parse positional parameters. optstring contains the option characters to be

recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated

from it by white space. The colon and question mark characters may not be used as option characters. Each time it is

invoked, getopts places the next option in the shell variable name, ini- tializing name if it does not exist, and the index of

the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each time the shell or a shell

script is invoked. When an option requires an argument, getopts places that argument into the variable OPTARG. The

shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same

shell invocation if a new set of parameters is to be used.

When the end of options is encountered, getopts exits with a return value greater than zero. OPTIND is set to the index of

the first non-option argument, and name is set to ?.

getopts normally parses the positional parameters, but if more arguments are given in args, getopts parses those instead.

getopts can report errors in two ways. If the first character of optstring is a colon, silent error reporting is used. In

normal operation diagnostic messages are printed when invalid options or missing option arguments are encountered. If

the variable OPTERR is set to 0, no error messages will be displayed, even if the first character of optstring is not a colon.

If an invalid option is seen, getopts places ? into name and, if not silent, prints an error message and unsets OPTARG. If

getopts is silent, the option character found is placed in OPTARG and no diagnostic message is printed.

If a required argument is not found, and getopts is not silent, a question mark (?) is placed in name, OPTARG is unset,

and a diagnostic message is printed. If getopts is silent, then a colon (:) is placed in name and OPTARG is set to the option

character found.

getopts returns true if an option, specified or unspecified, is found. It returns false if the end of options is encountered or

an error occurs.

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functionsfunctions

� install runnable code unit in memory

� under a callable name

“A shell function… stores a series of commands for later execution.

When the name of a shell function is used as a simple command name,

the list of commands associated with that function name is executed.

Functions are executed in the context of the current shell; no new process

is created to interpret them (contrast this with the execution of a shell script).

bash man page


functionsfunctions

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filenamesfilenames

� decomposition

– basename

– dirname

� temporary names

� random names


Filename decompositionFilename decomposition

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Generating working/temp filenames Generating working/temp filenames

““datedate”” basedbased


Generating working/temp filenamesGenerating working/temp filenames

RANDOM variable, RANDOM variable, mktempmktemp commandcommand

in /tmp

in some other

directory

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ifif--less ifless if

if [ . . . ]; then

xxx

fi

[ . . . ] && xxx

� seen often

� understood seldom

� a (too?) slick way to do conditionals


ifif--less ifless if

Some examples found in rc.sysinit

AND-based righthand action only if lefthand condition true:

[ -x /usr/bin/plymouth ] && PLYMOUTH=yes

[ ! -f "$file" ] && continue

[ "$READONLY" = "yes" ] && return 1

[ -f /var/log/dmesg ] && mv -f /var/log/dmesg /var/log/dmesg.old

OR-based righthand action only if lefthand condition not true:

[ -f /.autorelabel ] || touch /.autorelabel

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sourcesource

� execute code from a file in current shell

� often used on a file containing functions

� effectively similar to c language #include

“source filename [arguments]

Read and execute commands from filename in the current shell environment”

bash man page


findfind

� searches for files in a directory tree

� described by an expression

� expression consists of elements

– options

– tests

– actions

� each element returns boolean result

� find evaluates as many elements of its expression as needed to know expression’s outcome

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Most common useMost common use

for <all files in a set of files>

if <something about the file> do <something with the file>

next

but the operation details are more complex than that


find examplefind example

find . -maxdepth 1 -size +1000000c -print

expression

a testan option an action

find files 1) in the current directory (no subdirectory search)

2) bigger than a million bytes

3) and print their names

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Some example elementsSome example elements

find . options* tests actions

maxdepth name print

mount atime +n ls

etc size +n exec

executable ok

type etc

empty

false

etc

* find’s options, not shell command options


Some example elementsSome example elements

find . options tests actions

what it returns: true always true or false true or false

what it does: influence nothing their particular

find behavior action

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Operational logicOperational logic

"[evaluates] the given expression from left to right...

until the outcome is known (the left hand side is false for

and operations, true for or), at which point find moves on

to the next file name.“

- “find” man page


Operational logicOperational logic

because -name “A*” is false for B* files

because printing happens before -name “A*” evaluation

from the 2nd -print

(2nd print doesn’t happen for B* files)

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exec action exec action ––

arbitrary response for qualifying filesarbitrary response for qualifying files

a “finder” script command:

find . -type f –exec grep –l “$1” {} \;

print names of all files in current directory containing

a given string

� needs to be terminated with ;

� uses {} as placeholder for current file

� need to escape these from shell


String manipulationString manipulation

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sedsed –– stream editorstream editor

� used for search and replace

� filters standard input to standard output


sedsed –– stream editorstream editor

prints 2 lines replace 1st o with *, each line

all o’s

replace any h or e

replace any vowel

replace any letter or numeral

replace anything neither h nor e

replace anything neither letter nor numeral

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awkawk ((oror gawk)gawk)

� a pattern scanning and processing language

� better text processing facilities than shell’s

� often used in scripts to break text into fields


gawkgawk

gawk ‘ <program> ’ <input files>

{ <pattern> <action> }

gawk processes all lines in the input, comparing each to the pattern

and, for those that match, performing the action

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gawkgawk

the program

the pattern the action

pattern only: default action is to print whole line

action only: default pattern selects all lines


bcbc

� arbitrary precision calculator– strong computation features

– operates internally in decimal

� interactive– offers command prompt

� programmable– weak programming features

– complements shell (weak comp, strong prog)

� use with shell– pipe commands to bc from shell

– result returns to shell from bc

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bcbcexpressions

high integer precision

hig

h in

teg

er

pre

cis

ion

hig

h in

teg

er

pre

cis

ion


bcbc how shell does it

(no decimals)

how bc does it

high decimal precision

decimal periodicity = 1



hig

h d

ecim

al p

recis

ion

hig

h d

ecim

al p

recis

ion

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bcbc7, 97, 983 and other primes have special properties (see “cyclic numbers”)

periodicity = 7 – 1 = 6



hig

h in

teg

er

pre

cis

ion

hig

h in

teg

er

pre

cis

ion


bcbc -- functionsfunctions

enter function code at bc prompt

utilize it thereafter

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bcbc –– function function ““librarylibrary”” filesfiles

mod( ) – no such function

but here’s a file that defines

such a function (plus others)

file’s contained functions are now available


bcbc –– code from standard inputcode from standard input

things you do in bcthings you do in bc

you can have done by bc in the shell

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quick RSA tutorialquick RSA tutorial

� with suitably chosen values for e, n, and d*

� an integer m encrypts to: c= me mod n

� and can be recovered by: cd mod n

� a set of suitable values (ones that work):

e – 15941n – 48863

d – 17741

* pair {e,n} is called “public key” and pair {d,n} “private key”


encrypt and recover 1001 in encrypt and recover 1001 in bcbc

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encrypt and recover 1001 in shell scriptencrypt and recover 1001 in shell scriptuse command substitution where the command is use command substitution where the command is bcbc


use here doc to drop in entire use here doc to drop in entire bcbc progprog

any bc code that returns a

desired calculated value

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Looping helpersLooping helpers


debugging scriptsdebugging scripts

� sectional debugging

– turn on with “set –x”, off with “set +x” within

script

� whole-script debugging

– “set –x” at command line before running

script

– shabang line in script: #!/bin/bash -x

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ResourcesResources� Advanced Bash-Scripting Guide, An in-depth exploration of the art of shell scripting;

Mendel Cooper

� GNU manuals:find: http://www.gnu.org/software/findutils/manual/find.htmlgrep: http://www.gnu.org/software/grep/manual/sed: http://www.gnu.org/software/sed/manual/sed.htmlbc: http://www.gnu.org/software/bc/manual/bc.html

� Classic Shell Scripting Arnold Robbins and Nelson A. F. Beebe, O'Reilly & Associates, 2005

� UNIX Shells by Example Ellie Quigley; Prentice Hall, 4th edition, 2005

� Learning the bash Shell Cameron Newham and Bill Rosenblatt, O'Reily & Associates, 3rd edition, 2005

� Introducing Regular Expressions Michael Fitzgerald, O'Reilly & Associates, 2012

� Mastering Regular Expressions Jeffrey E. F. Friedl, O'Reilly & Associates, 3rd edition, 2006

� Sed and Awk Dale Dougherty, O'Reilly & Associates, 1997

� The AWK Programming Language, Alfred Aho, Brian Kernighan, Peter Weinberger, Addison-Wesly Publishing Company, 1988

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Shell Script Programming 2 - Santa Monica College