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Shell Script Programming 2 Useful Capabilities Shell Script Programming 2 David Morgan © David Morgan 2011-13 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 © David Morgan 2011-13 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 © David Morgan 2011-13 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 © David Morgan 2011-13 2 shift – positional parameter promoter © David Morgan 2011-13 getopts getopts vs getopt – a shell built-in – a stand-alone binary – not identical processes command-line options © David Morgan 2011-13 3 Command -line format command [ - options ] [ arguments ] ls -l m* tokens © David Morgan 2011-13 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 ) © David Morgan 2011-13 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 © David Morgan 2011-13 “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 © David Morgan 2011-13 functions © David Morgan 2011-13 6 filenames decomposition – basename – dirname temporary names random names © David Morgan 2011-13 Filename decomposition © David Morgan 2011-13 7 Generating working/temp filenames “date ” based © David Morgan 2011-13 Generating working/temp filenames RANDOM variable, mktemp command in /tmp in some other directory © David Morgan 2011-13 8 if -less if if [ . ]; then xxx [ . ] && xxx fi seen often understood seldom a (too?) slick way to do conditionals © David Morgan 2011-13 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 © David Morgan 2011-13 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 © David Morgan 2011-13 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 © David Morgan 2011-13 10 Most 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 © David Morgan 2011-13 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 © David Morgan 2011-13 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 © David Morgan 2011-13 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 © David Morgan 2011-13 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) © David Morgan 2011-13 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 © David Morgan 2011-13 String manipulation © David Morgan 2011-13 14 sed – stream editor used for search and replace filters standard input to standard output © David Morgan 2011-13 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 © David Morgan 2011-13 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 © David Morgan 2011-13 gawk 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 © David Morgan 2011-13 16 gawk the program the pattern the action pattern only: default action is to print whole line action only: default pattern selects all lines © David Morgan 2011-13 bc 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 © David Morgan 2011-13 17 bc bc high integer precision bc bc high decimal precision expressions how bc does it high decimal precision high integer precision decimal periodicity = 1 decimal periodicity = 2 decimal periodicity = 6 © David Morgan 2011-13 how shell does it (no decimals) © David Morgan 2011-13 18 bc bc high integer precision bc bc -- functions functions 7, 97, 983 and other primes have special properties periodicity = 7 – 1 = 6 utilize it thereafter enter function code at bc prompt periodicity = 97 – 1 = 96 periodicity = 983 – 1 = 982 (see “cyclic numbers”) © David Morgan 2011-13
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