Shell Script Programming 2 Useful Capabilities

Shell Script Programming 2

David Morgan

Useful 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

1 Supplemental 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 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

2 shift – positional parameter promoter

getopts

getopts vs getopt – a shell built-in – a stand-alone binary – not identical processes command-line options

3 Command -line format

command [ - options ] [ arguments ]

ls -l m*

tokens

Some options take a value, some commands take arguments command [ - options ] [ arguments ]

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

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

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

4 “getopts ” command

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

“getopts ” documentation (from “man 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. © David Morgan 2011-13

5 functions

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

functions

6 filenames

decomposition – basename – dirname temporary names random names

Filename decomposition

7 Generating working/temp filenames “date ” based

Generating working/temp filenames RANDOM variable, mktemp command

in /tmp

in some other directory

8 if -less if if [ . . . ]; then xxx [ . . . ] && xxx fi

seen often understood seldom a (too?) slick way to do conditionals

if -less 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

9 source

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

find

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

10 Most common use for

if do next

but the operation details are more complex than that

find example

expression

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

an optiona test an action

find files 1) in the current directory (no subdirectory search) 2) bigger than a million bytes 3) and print their names

11 Some 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 © David Morgan 2011-13

Some 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

12 Operational 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 logic

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

because printing happens before -name “A*” evaluation

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

13 exec action – arbitrary response for qualifying files

needs to be terminated with ; uses {} as placeholder for current file need to escape these from shell

a “finder” script command:

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

print names of all files in current directory containing a given string

String manipulation

14 sed – stream editor

used for search and replace filters standard input to standard output

sed – stream editor

prints 2 lines replace 1 st 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

15 awk (or gawk)

a pattern scanning and processing language better text processing facilities than shell’s often used in scripts to break text into fields

gawk

gawk ‘ ’

{ }

gawk processes all lines in the input, comparing each to the pattern and, for those that match, performing the action

16 gawk

the program

the pattern the action

pattern only: default action is to print whole line

action only: default pattern selects all lines

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

17 bc

expressions high integer precision

n n

i i

how shell does it bc (no decimals)

n n how bc does it

i i

ss i i high decimal precision cc

l l

i i

d decimal periodicity = 1 d

i i decimal periodicity = 2 hh

decimal periodicity = 6

18 bc 7, 97, 983 and other primes have special properties (see “cyclic numbers”)

periodicity = 7 – 1 = 6

n n

i i

cc periodicity = 97 – 1 = 96

i i

bc - functions

enter function code at bc prompt

utilize it thereafter

19 bc – function “library ” files

mod( ) – no such function

but here’s a file that defines such a function (plus others)

file’s contained functions are now available

bc – code from standard input

things you do in bc

you can have done by bc in the shell

20 quick RSA tutorial

with suitably chosen values for e, n, and d * an integer m encrypts to: c= m e mod n and can be recovered by: cd mod n

a set of suitable values (ones that work): e – 15941 n – 48863 d – 17741

encrypt and recover 1001 in bc

21 encrypt and recover 1001 in shell script use command substitution where the command is bc

use here doc to drop in entire bc prog

any bc code that returns a desired calculated value

22 Looping helpers

debugging 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

23 Resources

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.html grep: http://www.gnu.org/software/grep/manual/ sed: http://www.gnu.org/software/sed/manual/sed.html bc: 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