Fall 2006 Shell programming, part 2If example#!/bin/bash# 2006 09 08 - demonstrate if / then / elseif [ "x$1" != "x" ] && [ -f "$1" ]thenecho -n "Remove $1 (n)? "read answerif [ $answer == "y" ] || [ $answer == "Y" ] || [ $answer == "yes" ]thenecho "Would remove"elseecho "Would NOT remove"fielseecho "Please specify a regular file"fiCOP 4342Fall 2006 Shell programming, part 2If example#!/bin/bash# 2006 09 08 - demonstrate if / then / elseif [ "x$1" == "x" ]thenecho "Please specify a regular filename!"exit 1elif [ ! -f "$1" ]thenecho "$1 is not a regular file!"exit 1elseecho -n "Remove $1 (n)? "read answerif [ $answer == "y" ] || [ $answer == "Y" ] || [ $answer == "yes" ]thenCOP 4342Fall 2006 Shell programming, part 2echo "Would remove"elseecho "Would NOT remove"fifiCOP 4342Fall 2006 Shell programming, part 2The case statementcase WORD in PATTERN1 ) COMMANDS ;; PATTERN2 ) COMMANDS;; ... esacThe idea here is that WORD is tested against thevarious PATTERNs listed, in order. The first match thenexecutes the associated COMMANDs.COP 4342Fall 2006 Shell programming, part 2Case example#!/bin/bash# 2006 09 08 - case examplecase $1 in"yes")echo "Thanks!"exit 0;;"no")echo "Okay!"exit 1;;*)echo "Please use either ’yes’ or ’no’ (case-sensitive)";;esac;COP 4342Fall 2006 Shell programming, part 2While/until loopswhile list; do list; done;until list; do list; done;while executes the do list as long as the last commandin t he list returns 0. until executes until the lastcommand in the list returns 0.COP 4342Fall 2006 Shell programming, part 2while example#!/bin/bash# 2006 06 08 -- rdlecho -n "Now ’finish’ ? "read cmdwhile test $cmd != "finish"dorm NONEXISTecho "Status of \$? == $?"echo -n "Now ’finish’ ? "read cmddoneCOP 4342Fall 2006 Shell programming, part 2until example#!/bin/bash# 2006 06 08 -- rdlecho -n "Now ’finish’ ? "read cmduntil test $cmd == "finish"dorm NONEXISTecho "Status of \$? == $?"echo -n "Now ’finish’ ? "read cmddoneCOP 4342Fall 2006 Shell programming, part 2Shifting the argumentsYou can “shift” the argument list, eliminating thecurrent $1 and replacing it with the current $2, andso forth:COP 4342Fall 2006 Shell programming, part 2Shifting the arguments#!/bin/bashwhile [ $# -gt 0 ]doecho "$# --> arguments == ’$@’"shift;doneCOP 4342Fall 2006 Shell programming, part 2Shifting the arguments[langley@sophie 2006-Fall]$ ./Script8.sh a b c d e f g h8 --> arguments == ’a b c d e f g h’7 --> arguments == ’b c d e f g h’6 --> arguments == ’c d e f g h’5 --> arguments == ’d e f g h’4 --> arguments == ’e f g h’3 --> arguments == ’f g h’2 --> arguments == ’g h’1 --> arguments == ’h’[langley@sophie 2006-Fall]$COP 4342Fall 2006 Shell programming, part 2exitWe have already talked about exit, but to reiteratesome points about exit:☞ An exit status of zero should indicate success. It is agood idea to use an explicit exit NUM in scripts.☞ An exit status that is non-zero should indicate failure.☞ C programs use exit(NUM) to return a status.COP 4342Fall 2006 Shell programming, part 2exit example#/bin/bash# 2006 09 08 -- rdl Script9.shif ./Script10.shthenecho -n "Enter filename: "read filenameecho "You entered ’$filename’"elseecho "Okay, no filename needed."fiCOP 4342Fall 2006 Shell programming, part 2exit example#/bin/bash# 2006 09 08 -- rdl Script9.shwhile /bin/truedoecho -n "Should I ask for a filename? "read answercase $answer in"no")exit 1;;"yes")exit 0;;*);;COP 4342Fall 2006 Shell programming, part 2esacdoneCOP 4342Fall 2006 Shell programming, part 2Regular expressionsRegular expres sions are a c onvenient way to describe asequence of characters, and regular expressions are partof such programs as emacs, awk, and perl.COP 4342Fall 2006 Shell programming, part 2Regular expressions: operationsConcatenation: just place items adjacent, such ab, xyz,or somecharsCOP 4342Fall 2006 Shell programming, part 2Regular expressions: operationsRepetition: we use “*” to indicate repetition zero ormore times:a*b == b, ab, aab, aaab, ...COP 4342Fall 2006 Shell programming, part 2Regular expressions: operationsSpecial case of repetition: we can specify one or moretimes with +:a+b == ab, aab, aaab, ...COP 4342Fall 2006 Shell programming, part 2Regular expressions: characters and classesThe dot “.” can indicate any character, such asa.b == a1b, a2b, a3b, ...COP 4342Fall 2006 Shell programming, part 2Regular expressions: characters and classesTo specify a class of characters, you can use the [ ]syntax:[abc] == a, b, c[a-d] == a, b, c, d[^a-z] == NOT a lower case character[0-9] == 0, 1, 2, 3, 4, 5, 6, 7, 8, 9COP 4342Fall 2006 Shell programming, part 2AnchoringYou can “anchor” an expression to either the beginningof a string or its end, or both. Useˆto indicate thebeginning of a line, and $ to indicate the end:^abc$ matches a line that c onsists exactly of abcabc$ matches a line that e nds in abc^abc maches a lines that begins with abcCOP 4342Fall 2006 Shell programming, part 2Alternation and groupingYou can specify a group with round brackets “(“ and“)”.You can specify alternatives with a vertical “”(abc)|(def) matches either abc or defCOP 4342Fall 2006 Shell programming, part 2Note on groupingIt also possible in many instances possible to make areference to whatever matched a group in round brackets.COP 4342Fall 2006 Shell programming, part 2Check chapter 32 for more on regularexpressions32.20 has a good summary of metacharacters fordifferent programs.32.21 has a reference with many useful examplesCOP 4342Fall 2006 Shell programming, part 2Using grep/egrepYou can use the grep program to find strings in files.The “-i” option makes the search case-insensitive. If nofile or fi les are specified, then grep looks to stdin forinput. grep also adds “?” as a special character thatmatches 0 or 1 instance of any character.COP 4342Fall 2006 Shell programming, part 2Examples w ith grep/egrepegrep [Ll]angley * # finds instances of ‘‘langley’’ or# ‘‘Langley’’ in all files in the# current working directoryegrep -i she?p * # finds case-insensitive instances of# shep and she.pegrep -c /bin/bash * # shows filename and# number of matchesCOP 4342Fall 2006 Shell programming, part 2Popular options with grep/egrep☞ -i → case-insensitive☞ -c → display count of matching lines rather allmatching lines☞ -v → invert the matching☞ -H → always show filenames☞ -h → always suppress filenamesCOP 4342Fall