#2 Lab Find and Grep Due in Lab, September 8, 2004 Name: Lab Time: Grade: Find The nd command is used to locate les in a le system. The nd command starts at a single directory and descends recursively into all of its subdirectories to locate les. The nd command is very useful, but it has an obscure syntax that can be hard to understand. The format of the nd command is as follows: find path operators Type the following: find ~/csci3308 -print -print is an operator that prints the le name of a le. The directory ~/csci3308 says to start from that directory and apply -print to every le in every directory below it. This will print a lot of les, and could also be accomplished with ls -R. Now type this: find ~/csci3308 -name bin -print This will print only les and directories named bin in your directory structure. This is the purpose of nd: to locate les and directories that match specied characteristics. You can also search for wildcard expressions. Type the following: find ~/csci3308 -name \*.c -print This should print every le that ends with .c. The backslash is used to prevent the shell from interpreting the asterick in the above command line. Another way of writing the same command is: find ~/csci3308 -name *.c -print 1. Write a nd command to print all les under your csci3308 directory whose names are exactly three characters. /10
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Quoting in the Shell The quotes above are necessary because you want *.c to be interpreted by the nd program, and not the shell. Without the quotes the shell will process the * character and nd will never see it. Find knows how to interpret shell wildcards, but you need to keep the shell from replacing them before nd can see them. There are three types of quotes in tcsh, single quotes, , double quotes, ", and backslash, \. The purpose of quotes is to make the shell ignore characters that it would normally interpret or replace. Single and double quotes must come in matching pairs, and they aect everything between the quotes. Backslash aects only the single character immediately following the backslash. There are some minor dierences between single quotes and double quotes. Finding the right quotes for a particular situation can sometimes be a process of trial and error. Here is a brief summary of the dierences between single quotes and double quotes when using them at the command line. Single quotes protect all metacharacters from interpretation, with the exception of the history character (i.e. !). Metacharacters are characters which have special meaning to the shell like * or \. Double quotes protect all metacharacters from interpretation, with the exception of the history character (!), the variable substitution character ($), and the back quote, which is used for command substitution (see below). For a complete description of quoting-related issues, see the tcsh man page, or a book that covers Unix-related topics. 2. Type the following and record the results for each line next to it below: echo echo echo echo $shell $term $shell $term "$shell $term" \$shell $term
3. Why is the output of the rst and third lines the same?
In addition to quoting, there is the backward single quote, , also called back tick or back quote. The back quote invokes command substitution, and its purpose is to cause more substitution, not less. Anything between two back quotes is considered to be a command. This command is executed and its output is placed on the command line where the original command used to be. Try the following: echo date echo date 2
Back to Find -name and -print are just two operators, and nd has many more. Type the following: find ~/csci3308 -perm -004 -print This should nd all les in your csci3308 directory that have read permission for the others category. You can use this command to check what les in your directory are publically accessible to other users. 4. Rewrite the command above to locate those les whose permissions are exactly 004. Use the find man page for help.
Look at the nd operators in the nd man page. 5. Write a nd command that prints all les in your ~/csci3308 directory accessed less than 5 days ago. Try this command out to make sure it works.
6. Write a nd command that prints all les in your ~ directory that are of type directory, and whose names contain a numeric character.
Find can create more complicated boolean expressions with its operators. The exclamation point for stands not. The following command nds les underneath the current directory whose names do not end in .c. find . ! -name *.c -print
Sometimes parentheses must be used for grouping especially when using -a (and), or -o (or). The parentheses must be quoted with either quotes or a backslash so that the shell does not interpret them. find . \( -type f -o -type d \) -a \( -name program* -o -name bin \) -print 3
Find has two operators, -exec and -ok that can be used to run other programs on the les that are found. Try the two commands below: find . find . -type d -name bin -exec echo {} is cool \; -type d -name bin -ok echo {} is cool \;
7. What is the dierence between -exec and -ok?
In every -exec or -ok command you can use the two characters {}. When the command is run these characters are replaced with the name of the le being found. If multiple les are found the command is run once for each le, and each time {} is replaced with a new name. Also, any -exec or -ok command must end with a semicolon (;) which must be quoted, in this case with backslash. If the semicolon is missing, the shell will report a syntax error. Grep Grep searches the contents of a le to nd lines that match a pattern. Grep uses regular expression syntax for its pattern matching, which is dierent than shell wildcards. Once again, patterns have to be quoted to keep the shell from trying to evaluate them. In this lab we will be using egrep, expression grep, which is grep with a few extra features. In general, we will use the words grep and egrep interchangeably. In most cases, grep and egrep return the same results...we will try to highlight cases in which their output is dierent. For instance, the two commands below produce dierent output (You can look at the le /usr/dict/words if you want): egrep ^b.*(na)+$ /usr/dict/words grep ^b.*(na)+$ /usr/dict/words 8. What is the output for each command and why is it dierent?
Returning to our discussion, grep is dierent from nd in another way. Find only matches a lename if the entire lename matches a pattern. grep will display a line as a match if any part of the line matches an input pattern. Try this: egrep fly /usr/dict/words
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Grep nds not just the word y, but every word that contains the word y. If you want to match exactly your pattern and nothing more use the beginning and end of line characters, ^ and $. egrep ^fly /usr/dict/words egrep fly$ /usr/dict/words egrep ^fly$ /usr/dict/words Sometimes, you need to keep grep from interpreting special characters too. For example, you may want to nd all lines with matched parenthesis in your .cshrc le. You start by trying this: egrep (.*) ~/.cshrc the shell sees (.*) and it knows not to evaluate special characters in this word. It strips o the quotes and passes (.*) to egrep. However, egrep uses parentheses for grouping. egrep assumes you want to nd the pattern .*, and you put parentheses around it just to be careful. So egrep tries to match the pattern .* which matches anything and prints every line of your le. What you actually wanted was the literal character left parenthesis, (, then anything, .*, and then a literal right parenthesis, ). You need to put backslashes in front of the parentheses to make egrep treat them as literal characters. egrep \(.*\) ~/.cshrc So the single quotes are for the shell, and the backslashes are for egrep. You still need the single quotes or the shell would try to interpret the backslashes and asterisk. Sometimes getting the exact pattern you want can be tricky. Note: this is an instance where grep and egrep return dierent results. grep does not assign special meaning to parentheses so the parentheses do not need to be quoted when using grep. egrep has a more powerful regular expression engine than grep in which parentheses are assigned a special meaning, which we will cover later in the semester. 9. Write an egrep command to search a le named text for lines that contain a backslash. To test this create a sample le named text in your tmp directory with some lines that contain a backslash, and some that dont.
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Using Grep with Find Grep and nd can be combined to create a powerful search tool. Write a nd command, and then add the -exec operator to run a grep command on the les that it nds. The following command nds all .c les in your directory structure that contain the word help: find ~ -name *.c -exec egrep help {} \; -print
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