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9 The sed Editor

9 The sed Editor

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9 The sed Editor

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  1. 9 The sed Editor Mauro Jaskelioff (based on slides by Gail Hopkins)

  2. Introduction • sed is a Stream Editor • Designed to edit files in a batch fashion • Not interactive • Often used for text substitution • When you have multiple changes to make to one or more files: • Write down the changes in an editing script • Apply the script to all the files

  3. What does sed do? • Used to edit input streams • Input stream can be from a file, from a pipe or from the keyboard • Produces results on standard output • …but results can be put in a file or sent through a pipe

  4. Typical Uses of sed • Editing one or more files automatically • E.g. replace all occurrences of a string within a file for a different string • Simplifying repetitive edits to multiple files • E.g. perform the same operation on lots of similar files

  5. How Does sed Work? • Each line of input is copied into an internal buffer known as a “pattern space” • All editing commands in a sed script are applied, in order, to each line of input (in the buffer) • Editing commands are applied to all lines in the buffer • Unless line addressing is used to restrict the lines affected

  6. Furry caterpillars crawl slowly Furry spiders crawl slowly Furry spiders run slowly How does sed Work? (2) • If a sed command changes the input, the next command will apply to this new (changed) line of input, not the original one • More on this later! s/caterpillars/spiders/ s/crawl/run/ sed script Pattern space

  7. How does sed Work? (3) • When sed edits an input file, the original input file is unchanged • The editing commands modify a copy of each original line of input • When sed outputs the result, it is the copy that is sent to STDOUT (or redirected to a file) • sed keeps a separate buffer, known as the “hold space” • Can be used to save data for later retrieval • For most edits this isn’t needed - only if a command refers to it

  8. How to Run sed from the Command Line • sed [-n] [-e] ’command’ file(s) • For specifying an editing command on the command line • E.g.: • sed 's/ant/flea/g' myCreaturesFile • sed -e 's/ant/flea/g' -e 's/worm/slug/g' myCreaturesFile • (what does this mean??? - more about sed commands shortly…) • sed [-n] -f scriptfile file(s) • For specifying a scriptfile containing sed commands • E.g.: • sed -f myScript myCreaturesFile • If no file specified, sed reads from STDIN

  9. The -n flag • sed can be given a -n option • This tells sed NOT to write the contents of the pattern space by default to stdout: • sed -n 's/ant/flea/g’ myCreaturesFile • Another way of specifying this is to put #n at the start of a sed script • Why do we want to stop sed’s output? • We can then tell sed to print specific lines of output, rather than the whole pattern space: • sed -n 's/swan/coot/p’ myCreaturesFile • NOTE the p in the above example…

  10. sed Regular Expressions • sed uses regular expressions • The format of these is very similar to those used by grep

  11. sed Regular Expressions

  12. sed Regular Expression (2)

  13. sed Commands - Syntax • sed instructions consist of addresses and editing commands • They have the general form: • [address[,address]][!]command [arguments] • NOTE: here, [] denotes something is optional • Therefore: • If the address of the command matches the line of the pattern space (internal buffer), the command is applied to that line If ! is present then it means anything NOT in the address(es) stated Optional arguments to the command [address[,address]][!]command [arguments] Zero or more addresses The sed command to be executed

  14. sed Addresses • A sed command can have 0, 1 or 2 addresses • An address in a sed command can be: • A line number • The symbol $ (meaning the last line) • A regular expression enclosed in slashes (/regex /) • Therefore, an address can be thought of as “something that matches” in the pattern space

  15. sed Addresses (2) • If no address is specified: • The command applies to each input line • If one address is specified: • The command applies to any line matching the address • REMEMBER: an address can be a regular expression! • If two comma-separated addresses are specified • The command applies to the first matching line and all succeeding lines up to and including a line matching the second address • If an address followed by ! is specified • The command applies to all lines that DO NOT match the address

  16. sed Commands • Consist of a single letter or symbol • They tell sed to “do something” to the text at the address specified • E.g.: • s means substitute • g is a flag to the s command. It means global, or all occurrences of… (more on this later) • sed 's/ant/flea/g’ myCreaturesFile • …means substitute all occurrences of the word ant with the word flea in the file myCreaturesFile • …in this example, no address is specified and so sed applies the command to all lines in the pattern space

  17. sed Commands (2) • Another example: • sed -n ’/^squirrel/,/^swift/p’ myCreaturesFile • Print everything between the line starting squirrel and the line starting swift, inclusive • Here, there are 2 addresses, both are regular expressions: • /^squirrel/ • The first address is the first line matching “squirrel” at the start of the line • /^swift/ • The second address is the first line matching “swift” at the start of the line • REMEMBER: regular expressions are written between / and / • sed therefore prints between the first matching line (with squirrel at the start) and all succeeding lines up to and including a line matching the second address (with swift at the start)

  18. sed Commands (3) • An example using ! • sed ’/aardvark/!d’ myCreaturesFile • Delete any line that doesn’t contain the text “aardvark” in the file myCreaturesFile • An example using line numbers: • sed ’5s/wombat/womble/g’ myCreaturesFile • Substitute all occurrences of wombat with womble on line 5

  19. a, a, ants on my arma, a, ants on my arma, a, ants on my armthey’re causing me alarm! Putting more than one sed Element in a Command • An example of two elements together: • Input file: • sed -e 's/ant/flea/g’ -e ‘s/alarm/to itch/g’ myCreaturesFile • Output: a, a, fleas on my arma, a, fleas on my arma, a, fleas on my armthey’re causing me to itch!

  20. Putting more than one sed Element in a Command (2) • Input file: • sed -e ‘s/parrot/lizard/g’ -e ‘s/lizard/koala/g’myCreaturesFile • Output from sed: • Why??? At the top of the tree there were 4 parrots and 2 lizards At the top of the tree there were 4 koalas and 2 koalas

  21. …because • sed read in the line in the file and executed: • s/parrot/lizard/g • …to produce the text: • sed then performed the command: • s/lizard/koala/g • …on this new edited line to produce: • REMEMBER from previously: • If a sed command changes the input, the next command will apply to this new (changed) line of input, not the original one At the top of the tree there were 4 lizards and 2 lizards • At the top of the tree there were 4 koalas and 2 koalas

  22. Basic Editing a\ append text after a line c\ replace text i\ insert text before a line d delete lines s substitute y translate characters Line Information = display line number of a line p display the line l display control characters in ascii Input/Output Processing n skip current line and go to line below r read another file’s contents into the output stream w write input lines to another file q quit the sed script Yanking and Putting h copy into hold space; clear out what’s there H copy into hold space; append to what’s there g get the hold space back; wipe out the destination line G get the hold space back; append to the pattern space x exchange contents of hold space and pattern space Summary of sed Commands (4)

  23. Examples of commonly used sed Commands s sed ‘s/dog/cat/’ myfile substitute the first occurrence of dog with cat for each line found in myfile sed ‘s/dog/cat/g’ myfile substitute all occurrences of dog with cat in myfile sed ‘s/dog/cat/4’ myfile find every line in myfile with 4 “dog” strings and substitute the 4th occurrence of dog with cat on each sed ‘1,2s/dog/cat/g’ myfile substitute all occurrences of dog with cat in the first 2 lines of myfile ONLY sed ‘/dog/,/cat/s/.*//’ myfile look for the text dog followed by the text cat. Remove the lines containing them plus all text (possibly more than one line) in between. Repeat until end of file myfile. s/.*// means substitute all text found for an empty string

  24. Examples of commonly used sed Commands (2) d sed ‘1,2d’ myfile delete everything in myfile between line 1 and line 2 sed ‘5d’ myfile delete the fifth line from myfile sed ‘/^#/d’ myfile delete all lines starting with # in myfile p sed -n ‘/BEGIN/,/END/p’ myfile find a line containing BEGIN and print that line and all following lines up to and including a line containing END. Note: if there is no END, sed will still print all text after BEGIN due to its stream oriented nature - it doesn’t know there is no END until it gets to the end of the file!

  25. A flag to the s command. It tells s to substitute ALL occurences of… Flags to commands • sed commands can be given flags. We have already seen the substitute command with the g flag: • s/lizard/koala/g • Other flags to s are: • n - replace the nth occurrence of pattern with replacement text • e.g. sed ‘s/dog/cat/4’ myfile • p - print pattern space to stdout if substitution successful • e.g. sed -n ‘s/dog/cat/p’ myfile

  26. Flags to Commands (2) • w filename - write the pattern space of lines that are changed to resultsfile if substitution successful • e.g. sed ‘s/dog/cat/w resultsfile’ myfile • NOTE: here there must be exactly ONE SPACE between the w and the resultsfile • resultsfile will contain only those lines that sed applied the substitution to

  27. Running sed from a Script • sed commands can be put in a file called a script • E.g.: • …and run from the command line: # this is my sed script s/horse/cow/g s/chicken/duck/g s/newt/lizard/g A comment in sed script.sed $ sed -f script.sed myCreaturesFile

  28. Piping to and from sed(and a much more complicated example!) • The UNIX who command gives an output: $ who zliybbs pts/5 Apr 8 19:11 zliybsj2 pts/6 Apr 8 18:42 (ss-226-host39.nottingham.edu.cn) zliybyk2 pts/9 Apr 6 14:30 zliybyk2 pts/10 Apr 6 14:31 zliybbs pts/11 Apr 8 19:15 (10.20.50.15) zliybyy2 pts/12 Apr 8 20:10 zliybwj pts/15 Apr 6 14:34 zuczpd pts/17 Apr 6 14:44 zuczpd pts/18 Apr 6 14:44 zuczpd pts/19 Apr 6 14:44 (ss-226-host67.nottingham.edu.cn) zuczpd pts/20 Apr 6 14:45 (ss-226-host67.nottingham.edu.cn) zlizmj pts/1 Apr 9 08:49 (10.20.10.85)

  29. Piping to and from sed (2) (and a much more complicated example!) • If we wanted to extract only the machine names from this output, we could use the following command: • who | sed -n ‘s/.*(\(.*\))/\1/p’ What ON EARTH does this mean???? ☺

  30. Take the output from the UNIX who command and pipe it onto sed This denotes the start of a region of interest Take everything up to and including the first open bracket … who | sed -n ‘s/.*(\(.*\))/\1/p’ This denotes the end of a region of interest Take everything after the first open bracket “(“up to, but not including, the close bracket “)”and keep it for future referencing in a region of interest …and substitute it with the region of interest that was saved earlier, referenced as number 1 (REMEMBER from earlier: \n means nth group)

  31. Piping to and from sed (3) • If we then wanted to sort the result into alphabetical order, we could pipe it onto sort: who |sed -n ‘s/.*(\(.*\))/\1/p’ | sort • We could then redirect the whole output to a file: who | sed -n ‘s/.*(\(.*\))/\1/p’ | sort > machines.txt

  32. An Example of Data Manipulation using sed • Suppose we had a file names.txt in the form forename:surname (with a colon in between): • …and we wanted to reverse the names so that they were in the order surname,forename (with a comma in between)… Steve:Bradford Saun:Higgins Gail:Hopkins Sara:Mead Fred:Smith Henry:Taylor

  33. An Example of Data Manipulation using sed (2) • sed -e ‘s/\(.*\):\(.*\)/\2,\1/’ • …would produce the following output: EXPLANATION: This uses regions of interest. It puts the forename in a region of interest and then puts the surname in another region of interest. It then outputs the second region of interest followed by the first. Bradford,Steve Higgins,Saun Hopkins,Gail Mead,Sara Smith,Fred Taylor,Henry

  34. Using Different Delimiters • Often, / is used in sed scripts as a delimiter • However, other characters can be used as delimiters instead • sed takes the first character that it expects to be the delimiter as the delimiter • All of these are therefore equally viable: • Why would we want a different delimiter? s/horse/cow/g s,horse,cow,g s:horse:cow:g s$horse$cow$g

  35. Using Different Delimiters (2) • Suppose we had an HTML file which we wanted to convert to XHTML • We therefore want to change • all occurrences of <H1> to <h1> • all occurrences of <H2 to <h2> • all occurrences of </H1> to </h1> • all occurrences of </H2> to </h2> • and so on… s/<H1>/<h1>/g s/<H1>/<h1>/g s:</H1>:</h1>:g s:</H2>:</h2>:g … Here we have used : as a delimiter because there are slashes in the data

  36. sed Tries to Match the Longest Expression! • Suppose we had an HTML file and we wanted to remove all the markup: • We could instruct sed to find a ‘<‘ character followed by zero or more other characters until a ‘>’ character: • sed -e 's/<.*>//g' UST.html • This would produce: <b>Welcome</b> to the <i>UST</i> website. website. Why??

  37. sed Tries to Match the Longest Expression! (2) • …because sed tries to find the longest expression that matches: • <b>Welcome</b> to the <i>UST</i> • …instead, we need to specify that sed looks for a ‘<‘ character followed by zero or more non-‘>’ characters followed by a ‘>’ character: • sed -e 's/<[^>]*>//g' UST.html • sed will then match <b> and </b> and <i>, and so on…

  38. Character Classes - POSIX Compliant sed • Often in sed you want to specify a regular expression that contains white space (TABs, spaces, etc.) • POSIX compliant sed offers a simple way of doing this with a character class: • sed ‘s/[[:space:]]//g’ myfile • Character classes give you a way of specifying, within a regular expression, types of characters to search for

  39. [:alnum:] Alphanumeric [a-z A-Z 0-9] [:alpha:] Alphabetic [a-z A-Z] [:blank:] Spaces or tabs [:cntrl:] Any control characters [:digit:] Numeric digits [0-9] [:graph:] Any visible characters (no whitespace) [:lower:] Lower-case [a-z] [:print:] Non-control characters [:punct:] Punctuation characters [:space:] Whitespace [:upper:] Upper-case [A-Z] [:xdigit:] hex digits [0-9 a-f A-F] Character Classes (2)

  40. Summary • An introduction to sed • Format of sed statements • Addresses • Types of command • Putting sed inside a script • Some more advanced examples of sed