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This tutorial explores the concepts of pipes, file redirection, and pattern matching using regular expressions in Unix-like systems. You will learn how to connect commands using pipes, redirect standard input/output, and differentiate between wildcards and regular expressions. The exercises provided will guide you through finding files, handling command errors, and utilizing grep for text searching with regular expressions. By the end of this tutorial, you should be able to efficiently manage files and execute powerful text matching operations in the command line environment.
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Compsci215 Tutorial 3 • Pipes • Pattern matching-regular expression • Finding files • Exercises
Pipes • Standard input • Either a file or a standard output from another command • Standard output • Normal command output • Standard error • Errors or exceptions • A standard error does not stop executing rest of commands
Pipes • Command 1 | command 2 • Standard output from command 1will become standard input for command 2 • Standard error will be accumulated, it will be printed at the end or could be redirected to a file or /dev/null
Pipes jren042@wintermute02:~/compsci215$ ls err file1 output jren042@wintermute02:~/compsci215$ cat file1 file2 this is file1 cat: file2: No such file or directory jren042@wintermute02:~/compsci215$ cat file1 file2 1>output 2>err jren042@wintermute02:~/compsci215$ cat output this is file1 jren042@wintermute02:~/compsci215$ cat err cat: file2: No such file or directory
Pipes • File redirection • Use >< for file redirection • Use file as an input source or output • For input, use < • For output, use > • Pipes • Use | • Two or commands, • use previous command’s output as its input • Could use file redirection in pipes
Pattern matching-regular expression • Wildcard is different from regular expression • Wildcard is not as powerful as regular expression • Wildcard is good enough when specifying files in a directory or good enough when searching file content
Wildcard vs. Regular Expression • Wildcard • Expanded by the shell • Regular expression • It is a programming language • Expanded by other program Eg. grep, egrep Or perl, Java
Wildcard • * (asterisk) specifies zero or more characters to match • ? Match any single character • [...] match a set a characters • [abc] match a, b and c • [a-z] match characters from a to z
Regular Expression • . match any single character • Eg: he., will match hex, hel, he8, but not he88 • ^ state beginning of the line • If we want to get the text start with “what” • ^what • $ state end of the line • If we want to get the text end with “you” • you$ • If we want to get the text end with “?” or “.” • \?$, \.$
Regular Expression • [...] match a set of characters • [7-9], will match 7, 8 and 9 • [1, 9], will match 1 and 9 • [^...] match a set of complements characters • [^x-z], will match all characters other than x, y and z • [^a-zA-Z], will any non-alphabetic character • * will match the preceding character zero or more times • + will match the preceding character one or more times • ? Will match the preceding character zero or one time
Regular Expression • Match a precise number of characters: \{min, max\} • Match a number 4 times • [0-9]\{4\} • Match a character 10 times • [a-zA-Z]\{10\} • Match a single character 4 to 7 times • .\{4, 7\}
Regular Expression • Special characters in regular expression • ., *, +, [, \ • Use backslash to escape it • \d match a digit • \D match a non-digit • \s match a character and space • \S match a character and no space • \w match an alphanumeric character and “_” • \W match a non-alphanumeric character and no “_”
Regular Expression Example • How to match any txt that have “hello.txt” • hello\.txt • Hel* will match? • He, Hel, Hell, .... • Hel+ will match? • Hel, Hell,.... • .* will match? • Null, and any number of characters • How to match any txt start with “where” • ^where
Regular Expression Example • [a-z]*[12] will match • Any text have zero or more alphabet, followed by 1 or 2 • ^Ju.y$ will match • July, Ju3y, Juxy, but not Juy, Jully
grep and egrep command • egrep is more powerful than grep • It is the same as grep –E • Allow you to search within a file or files • grep pattern file(s) • Pattern should surrounded by single quotes • grep -n • Will show the line number and matched text
Interpreting Quotes • Single quote • stops wildcard expansion and variable substitution • Ignore dollar sign, back quote, and backslashes • Double quote • stops wildcard expansion • Back quote • runs the quote statement and replaces quotation with the output
Finding Files • Everything is a file in Unix • How to find a file in unix? • find [path_list] [selection_criteria] [action] • Will looking for any file that match the selection criteria in the path specified and its sub directories • Here the pattern is sued by shell, so wildcard • -type x, will match the file type • -user x, if owned by user x • Action could be print, ls or exec command
Finding files • Executing commands • -exec command {} \ • find ~ -name *.important cp {} {}.backup \; • Find directories with name start with compsci • find . –name compsci* –type d
xargs command • Find with –exec • Time consuming • If we find 200 files and want to remove them, -exec command have to be executed 200 times • With xargs, only need to be executed once • Use pipes • find . –name *.backup –print | xargs rm
Exercises • How to remove all files with extension backup • rm *.backup • How to remove a file called “*.backup” • rm “*.backup”
Exercise • find . –name “ls” –print • find . –name “.bak” –print • echo * • ls ???
Exercise • $ date • Sat Aug 19 17:14:53 NZST 2006 • give a shell pipeline to print the minutes • date | cut –d”:” –f2 • give a shell pipeline to print the second line of a file called mockingbird.txt • head -2 <mockingbird.txt | tail -1 • the file /usr/share/dict/words contains an words per line, how many words start with a vowel (a, e , i, o u) • cat /usr/share/dict/words/ | grep ”^[aeiou]” -print
Exercise • give a shell command that make file “test.sh” executable by everyone in your group • chmodg+x test.sh • list all the files in the current directory contains the word ls • ls *ls* • use find to list all the files(no directories) in the home directory and its sub-directories • find ~ -type d
Exercise • a=11, b=22, f=$a$b • c=‘echo $a$b’, single quote • d=`echo $a$b`, back quote • e=“echo $a$b”, double quote • output of • echo $c, • echo $a$b • echo $d , • 1122 • echo $e , • echo 1122 • echo $f , • 1122
Exercise • ls *.c • ls a?c • ls */*.c • ls “*.c” • ls ”*.*” • ls */a*
Exercise • output of following regular expression • a|b*, • null, a, b, bb.... • (a|b)*, • null, a, b, ab, ba, aa, bb, aba.... • L+, • L, LL, LLL.... • L?, • null, L • H.?e • He, Hoe, Hue,...
Exercise • write a regular expression that • only match 4 characters • ^....$ • start with two digits and end with a question mark • ^[0-9]{2}*\?$ • everything that not start a digit • ^[^0-9]* • containing only 3 digits • ^[0-9][0-9][0-9]$ • Files with 3 letter extension • *\.[a-zA-Z]{3}$ • how about the standard car registration plate? 3 letters followed by 3 digits or 2 letters followed by 4 digits.
