Systems Programming/ C and UNIX
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1 Systems Programming/ C and UNIX December 7-10, /17 December 7-10, / 17
2 Outline 1 2 Using find 2/17 December 7-10, / 17
3 String Pattern Matching Tools Regular Expressions Simple Examples of Quotes, Escapes, Alternatives, Sequences, and Wild Cards 3/17 December 7-10, / 17
4 String Pattern Matching Tools Three popular (and very useful) utilities let you search text for lines or strings that match a regular expression: grep is a very useful command to locate files with a particular characteristic. The name came from usage in ed, the old Unix text editor, where you wrote: global / regular expression / print emacs is the Free Software Foundation text editor used by many experts. It is more powerful and more complex than vi, the Unix editor and much more flexible than Word and similar editing applications. Emacs is especially useful when dealing with non-printing characters and editing data listings that result from running other Unix commands. sed (stream editor) is a filter used for reading a stream of text, identifying and replacing patterns, and writing the result. The file based version, ed is similar and still exists. 4/17 December 7-10, / 17
5 Regular Expressions In the context of grep, a regular expression is a pattern consisting of: Characters that must exactly match the text being searched.. which matches exactly one character of any sort. * is a postfix operator that is applied to the previous pattern element. The combination matches zero or more repetitions of the pattern element. Matching can be either unanchored or anchored. If the grep pattern is matched, the whole line is sent to stdout. A pattern can be anchored to either end of the line or be unanchored. If a pattern starts with ^ the match must start at the beginning of the line. If a pattern ends with $ the match must end at the end of the line. A match for an unanchored pattern can be anywhere on the line. 5/17 December 7-10, / 17
6 Grouping in Regular Expressions Alternatives and sets of characters can be specified: [ ] are used to group alternative characters or ranges of characters. A match is found if the input text matches any one of the alternatives. [a-z] means the set of characters from a to z. ( ) are used to surround parts of a pattern, with the meaning that whatever matches that part of the pattern is supposed to be captured and saved for reuse later in the pattern. This enables us to identify lines that have repeated parts. 6/17 December 7-10, / 17
7 Simple Examples of. The first line is used in the filter mode. The others are not. grep -n sock grep.txt > sock.out Find all lines in grep.txt that contain the word sock. Output the line number and the line to sock.out grep -i sock L8.tex Do a case-insensitive search. grep -r sock. Recursively search for lines containing sock in all the files in the current directory and all subdirectories. grep sock clientmary.c client.c You can list more than one file to search. grep sock../*.c You can use.. or specify files with a glob pattern, which will be expanded by the shell. 7/17 December 7-10, / 17
8 Quotes Use quotes if there are spaces embedded in your search pattern: grep sock L8.tex Find all lines containing the word sock (note the trailing space) in the file L8.tex grep "sock " L8.tex Double quotes are like using single quotes except for shell expansion rules. grep "sock s " L8.tex Two styles of quotes allow you to include a quote as part of a string. 8/17 December 7-10, / 17
9 Escape Characters and Options Sometimes you need escape characters: grep big\ cat L8.tex You can use backslash-space to put a space into a pattern. grep L8\. clientmary.c If you need to search for a period, the period must be preceded by a backslash (escape). Use -e when searching for multiple patterns. grep -v -e foo -e bar myfile.txt Use -v to search for lines that do NOT contain a pattern. This would find lines that contain foo but not bar grep -e foo -v -e bar myfile.txt 9/17 December 7-10, / 17
10 Alternatives and Sequences Sometimes you want to search for any character from a given set. To do this, put the set inside square brackets and quote the whole pattern. grep [qxz] test.txt Find lines that contain one of these three uncommon letters. grep -i [qxz][aeiou] test.txt Search for an unusual letter followed by a vowel. grep [v-z] test.txt Use a minus sign to define a range (in the local char set) of chars. grep -i [a-z][a-z] test.txt Search for two letter words preceded and followed by spaces. grep -i [a-z][0-9] * Case insensitive search for a letter followed by a digit. 10/17 December 7-10, / 17
11 Wild Cards Using *, you can search for zero, one, or more copies of anything. Any pattern that contains an * must be quoted; otherwise, the shell will think it is a glob pattern and try to expand it. grep c.t$ *.txt This matches cat, cot, and cut at the end of a line. grep c.*t *.txt This matches ct, cat, caat, caaat, cot, coat, coats,.... grep ^c*t *.txt This matches any line that starts with zero or more c s followed by a t. grep ac*t *.txt This matches at, act, action, acct, acctive, acccctoo, /17 December 7-10, / 17
12 Using find Using find Basic find Simple Examples of Using find Examples of find-combinations 12/17 December 7-10, / 17
13 Using find Basic find grep looks at the contents of a file; find looks at the meta data. The find command will do a recursive traversal of a directory, looking for files that match some criterion. find. -name "*.log" Search this directory for files with the extension.log. Display the relative pathname. find. -name "*.log" -ls Display the full directory listing, not just the pathname. find. -name "*.log" > logfiles.txt Redirect the listing to a file. Often used when you need to edit and format the information. find /sysprog/lecture -name "*.tex" You can use a pathname, relative or absolute. find.. -name "L*.pdf" You can search using a pattern. 13/17 December 7-10, / 17
14 Using find Examples of Using find Varied criteria can be used. find -empty Search for empty files and directories starting from my home directory. find -type f List all my files (not links or directories or... ). find -type l Search for symbolic (soft) links. find -type d Search for directories. find. -mtime -1 -type f List all the ordinary files that have been changed in the last day. find. -mmin -10 -type f List all the files that have been changed in the last 10 minutes. 14/17 December 7-10, / 17
15 Using find Using find Starting with my home directory, find all files bigger than 100*1024. find -type f -size +100k -ls Find all really big files. I tried +100M also. find -type f -size +1G -ls List all files that have more than one link. Use long form of listing. find. -links +1 -type f -ls Find all file names that link to the same Inode as the named file: find. -samefile./stu_sysprog/p2/test1/filec.dat 15/17 December 7-10, / 17
16 Using find Using find-combinations Basically, find() will traverse your file directory looking for anything you can figure out how to describe, then do anything you can find a command to do. Find all my log files and write their names to a file. find. -name "*.log" > logfiles.txt Search for files that end in build/debug or build/debug find. -iregex ".*build/[dd]ebug" Display the number of files in this directory tree, and the number of words and bytes in those file names. find. -type f wc Output: Find all the files in this directory tree with spaces in the path name: find. -type f grep " " 16/17 December 7-10, / 17
17 Using find Using find-combinations By combining find with other commands, you can do some fancy things: Starting here, find all files with names that ends in.tex. Then grep each of those files for the word socket. grep -i "socket" find. -type f -name "*.tex" Find all my directories that can be written by anybody find -perm +22 -type d Find all my files that can be written by anybody. For each one, use exec to call the chmod command to turn off the other write permission. The {} and + tell exec to run chmod on all the files identified by the find. find -perm +o=w -type f -exec chmod o-w {} + 17/17 December 7-10, / 17
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