TweetFollow Us on Twitter

Not CurSED, BlesSED!

Volume Number: 22 (2006)
Issue Number: 2
Column Tag: Programming

Mac In The Shell

Not CurSED, BlesSED!

by Edward Marczak

But sometimes, the keys to sed can be disguised.

This month, we're going to follow up on sed, the powerhouse non-interactive editor, introduced in part 1 in December's issue. Hopefully the holidays of that month didn't take you away from really digging in and putting the lessons into practice. If you did take it all in, you've probably found several uses for it already. This month, we'll visit more sed mnemonics and add a few new tricks.

Bob's Yer Uncle

A few reminders about sed regarding redirection and piping. In part 1, we worked on a single file, viewing the results on screen. There are some things I need to clear up before continuing.

Naturally, you can redirect the output from sed to a file. What I didn't mention last month is a tendency that many first-time sed users need to stay away from. Namely, redirecting to the same file that you're working on. Sure, you've tested your script and you're 100% confident that it's going to Do The Right Thing. So, you do this:

sed -f myscript.sed long_text_file.txt > long_text_file.txt

Seems perfectly natural, right? We'll, sorry, you can't do that. You can't redirect to the same file you're altering. This is not an issue with sed, but just the way the shell works. The shell will truncate the destination file before it executes your command. So, while it seems like it might work, it doesn't. You need to write to a temporary file first, and then overwrite the original if you desire to do so. The desired effect is achieved like this:

$ sed -f myscript.sed long_text_file.txt > tmp_file.txt
$ mv -f tmp_file.txt long_text_file.txt

Also, it may not have been clear from the introductory article that sed is happy to work 'on-the-fly' by accepting and pumping out data via a pipe. To get a count of directories in a certain folder, for example, you could do this:

ls -lRF /Users/marczak/Documents | grep "^/" | sed s/:/'\/'/ | sed s/\ /'\\ '/g | wc -l

Pipe-to-pipe-to-pipe-to-pipe! However, a sed master will never use two sed statements where one would do. This can also be written as:

ls -lRF /Users/marczak/Documents | grep "^/" | sed -e s/:/'\/'/ -e s/\ /'\\ '/g | wc -l

In short, pipe away, redirect carefully, and Bob's yer uncle!

Do it Better

While the information in part 1 of this article is more than enough for very powerful manipulations, you still may run into some limitations. That's why part 1 was the introduction: there's still more!

First, I mentioned that I would give the solution the 'swapping Bill and Michael' problem. The answer, of course, is, "it depends." You really have to be familiar with your source material. For now, I'll show the easy, yet most brute-force way of handling our scenario. Remember, the source text is this:

    Bill and Michael went to the store. Bill needed to buy some butter, eggs and flour. He and Michael were in a hurry to bake a cake for their parent's Anniversary. Once they got home, Bill and Michael realized that they forgot cake icing.

Just like writing any code, you can swap using a temporary variable (sorry - sed doesn't quite have anything like a bitwise swap!). So, here you go:

sed -e 's/Bill/David/g' -e 's/Michael/Bill/g' -e 's/David/Michael/g' short_story.txt

In our case, we probably really only want the fully qualified "Bill and Michael", so we can actually do just that:

sed 's/Bill and Michael/Michael and Bill/g' short_story.txt

However, you may realize, that this does not take into account line endings. What if our phrase crosses a newline boundary? Ah! That's where multi-line commands come in.

Multi-line commands give sed the ability to look at more than one line in the pattern space. This gives the sed script-crafter the ability to inject a little logic into the flow. Since our story does not split "Bill and Michael" anywhere, let's look at something that does: "buy some...". If we wanted to change all occurrences of "buy some" to "purchase some", even if it spans lines, we need to coax sed into doing so. Again, the brute-force way is simply this:

/buy/ {
N
s/buy *\n*some/purchase some/
}

Here, we look for the address "buy" and when found, run a multi-line next ("N") command. This command reads the next line of input and appends it to the current pattern space - still 'separated' by a new line. Like I said, brute-force, and doesn't really scale well. Also, this has the potential of outputting some really long lines. Of course, elegance is just around the corner. A script that gives us use in more general cases could look like this:

/buy/ {
N
s/ *\n/ /
s/buy some */purchase milk,\
/
}

First, we look for the address "buy", and if we find it, we pull the next line into the pattern space with "N". Then, we can ditch the new line character and replace it with a space. From there we can try to match our patterns. However, even this is a little problematic - just a drop. The example just given works just fine, but let's alter our story and script. Adding a line to the story to make it this:

    Bill and Michael went to the store. Bill needed to buy some butter, eggs and flour. He and Michael were in a hurry to bake a cake for their parent's Anniversary. Once they got home, Bill and Michael realized that they forgot cake icing. It is important that they had not forgotten anything for the special day.

And changing the script to this:

/got/ {
N
s/ *\n/ /
s/got home */returned to their abode\
/
}

...will lead to problems. The goal of this script is to change all occurrences of "got home" into "returned to their abode". Run it and see what happens. See the problem? "got" matches "forgotten" on the last line, but makes no substitutions, so the script quits without outputting that line. It's just MIA! What to do? Exempt the last line of the script. Change the "N" to "$!N" - sed recognizes "$" as the last line (not EOL like regexp).

The reality is that you'll find many, many, many examples like this. Depending on your source(s), you may only be able to make a script just so general. This goes back to the rule: test, test, test! You can't test your script enough.

Loose Fit

In addition to the main pattern space that sed matches and manipulates, there is also a hold buffer. The commands are pretty self explanatory: 'x' will eXchange the pattern space with the hold buffer. 'h' will copy the current pattern space into the hold buffer, overwriting what was being held previously. 'H' will do the same, but append to the current hold buffer. 'g' gets the contents of the hold buffer and replaces the pattern space. 'G' gets the contents of the hold buffer and appends its contents to the current pattern space.

Before I get into these commands, please remember that sed certainly is a descendent of the phrase TIMTOWTDI - There is more than one way to do it. Many times, there will be multiple solutions to the particular problem you're trying to overcome. Build up one piece at a time, test, and for goodness sake, document your solution! (Did I mention that sed scripts recognize "#" as a comment?)

So, let's say we want to print the line before and the line after our match, so we can see it in context - like grep's 'A', 'B', and 'C' flags. Here's one way to approach this:

sed -n '
'/Anniversary/' !{
# lines that do not match what we're looking for - save
x
# clear the current pattern buffer with delete
d
}
'/Anniversary/' {
# lines that match
# get the previous line from the hold buffer
x
# print it with p
p
# get the current line back from the hold buffer
x
# print that
p
# get the next line
n
# print it
p
# finally, drop this line into the hold buffer
x
}' short_story.txt

Going back to our short story, this will look for a line containing 'Anniversary', print the line before it, the line itself, and the line following. Note the use of the "-n" switch passed into sed. This switch tells sed to not print all output by default. Otherwise, you'll still see all of your input as it filters through sed. Of course, to make all of this more useful, you could drop this right into a shell script, and use $1 for the pattern - this would give you a generic script that will always perform the equivalent of "grep -C 1 pattern file.txt". Just remember: the way I broke this up over several lines is very bash specific. csh users must use the backslash to tell the interpreter that the line continues on.

Step On

Earlier, I alluded to sed as a programming language by mentioning the classic temp-variable-swap. Well, sed tends to be more full featured than most people realize. You can even implement flow-control! sed features two commands that let you control the logic of your script. 'b' branches to a label. (Reminds me of my favorite Motorola Assembly mnemonic - BRA - BRanch Always). One example, and you'll get it. This script is an alternate to the script just presented that emulates 'grep -C 1':

# find our pattern? jump!
/Anniversary/ b printit
# else hold it
h
# jump to end of script
b
:printit
# get previous line from hold buf
x
# print it
p
# get current line back
x
# print it
p
# get next line
n
# print that
p

First to note is the label. A label starts a line with a colon, and should contain seven characters or less. While most modern implementations allow a label to be any length - and is actually the POSIX spec - there are still some versions of sed that restrict a label to 7 characters max. With 'b', we just branch - make the jump. Another flow-control command is 't', or, test. Test allows us to branch conditionally. The jump only happens if the previous substitution was successful. Another example is in order. Imagine a file that lists a userclass by number, and it should be by name. Additionally, you must process the file differently for each userclass. Here's a mock script that could handle this:

/userclass/{
s/2000/executive/
t excpath
s/1500/management/
t manpath
s/1000/staff/
t stpath
# default action
=
b
:excpath
...
b
:manpath
...
b
:stpath
...
# end of script

Those examples should get you going with flow control in sed. Remember, you can certainly jump to a label ahead of your test and even get into (basic) recursion!

Harmony

There are a bunch of miscellaneous things that I'd like to point out before we wrap up our conversation about sed. Some of these fall into the "you have to know the rules before you can break them" category, so I waited to present them.

All of the examples I've given, and most that you'll see, use a forward slash ("/") as the delimiter. Amazingly, sed lets you use whatever you like. The delimiter is great if you have a very simple replacement, such as sed s/one/two/g. However, if you're replacing file paths, that could get messy. So, use an underscore, if you like: sed 's_/usr/sbin_/usr/bin_'. Easier to read, right?

The "=" (equal sign) mnemonic prints out the current line number. So, you can simply line number any text file:

sed '=' textfile.txt

Or, you can use to just find the lines that the pattern you're looking for lie in:

sed -n '/Bill/ =' story.txt

You've probably picked these two up by now, but I should make them clear. Quoting: you really only need to quote if you have metacharacters, but it's a good habit to get into. This: sed s/Bill/Mike/g is the same as this: sed 's/Bill/ Mike/g'. However, try this: sed s/.*address*\ (astring\)\(\1)$1)/ and sed is just going to cough up electrons. The second thing is the use of the "-e" switch. If you only have one command, you can forgo the "-e". If you have multiple commands, you need to add each of them to the list of editing commands with the "-e" switch.

Lastly, a note about newlines, or, EOL. Somewhat confusingly, you match a newline with the standard regexp '\n'. However, to output a newline, you use a literal newline:

sed 's/Bill/Bill\
/g' story.txt

This will drop a newline after every occurrence of 'Bill' in our sample text.

Cut 'Em Loose Bruce

...and I thought I was going to get to awk this month! Hopefully, this gives you some ideas about sed and its power. I also hope with practice, that you use this power. You really have to see sed as an editor. It just happens to be one that you don't use interactively like vi, emacs or pico. Go forth and edit non-interactively! sed is an indispensable tool for any system administrator's toolchest, and there are plenty of repetitive tasks waiting for you to automate.


Ed Marczak owns and operates Radiotope, a technology consulting company. Despite being around the technology block once or twice, he's thankful that there's always something new to look forward to. Something new at http://www.radiotope.com

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

NetShade 8.3 - Browse privately using an...
NetShade is an anonymous proxy and VPN app+service for Mac. Unblock your Internet through NetShade's high-speed proxy and VPN servers spanning 17 countries. NetShade masks your IP address as you... Read more
Adobe Animate CC 2020 20.0.1 - Animation...
Animate CC 2020 is available as part of Adobe Creative Cloud for as little as $20.99/month (or $9.99/month if you're a previous Flash Professional customer). Animate CC 2020 (was Flash CC) lets you... Read more
Adobe Acrobat DC 19.021.20058 - Powerful...
Acrobat DC is available only as a part of Adobe Creative Cloud, and can only be installed and/or updated through Adobe's Creative Cloud app. Adobe Acrobat DC with Adobe Document Cloud services is... Read more
Adobe Acrobat Reader 19.021.20058 - View...
Adobe Acrobat Reader allows users to view PDF documents. You may not know what a PDF file is, but you've probably come across one at some point. PDF files are used by companies and even the IRS to... Read more
Adobe Flash Player 32.0.0.303 - Plug-in...
Adobe Flash Player is a cross-platform, browser-based application runtime that provides uncompromised viewing of expressive applications, content, and videos across browsers and operating systems.... Read more
Adobe InDesign CC 2019 15.0.1 - Professi...
InDesign CC 2019 is available as part of Adobe Creative Cloud for as little as $20.99/month (or $9.99/month if you're a previous InDesign customer). Adobe InDesign CC 2019 is part of Creative Cloud.... Read more
Adobe Lightroom Classic 9.1 - Import, de...
You can download Lightroom for Mac as a part of Creative Cloud for only $9.99/month with Photoshop, included as part of the photography package. The latest version of Lightroom gives you all of the... Read more
Shredo 1.2.7 - $6.99
Shredo is a beautiful, functional file-shredding and privacy scan utility. It permanently shreds files, folders, and external volumes' contents to keep information secure and impossible for anyone to... Read more
Visual Studio Code 1.41.0 - Cross-platfo...
Visual Studio Code provides developers with a new choice of developer tool that combines the simplicity and streamlined experience of a code editor with the best of what developers need for their... Read more
calibre 4.6.0 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more

Latest Forum Discussions

See All

King's Throne, the hugely ambitious...
King's Throne: Game of Lust is a deeply immersive medieval-set idle RPG which sees you playing as an ambitious prince, and sole heir to your father's kingdom. On a seemingly ordinary night whilst wandering the king's castle, you make the shocking... | Read more »
Abyssrium Pole is an upcoming aquarium b...
FleroGames' upcoming Abyssrium Pole has recently hit one million pre-registers, which is very impressive, particularly for a fairly casual looking game. Those who have pre-registered will receive 1000 Pearl when the game launches on 8th January... | Read more »
Two Spies is pretty fun, but it's h...
Two Spies just dropped on the App Store this week, and it looks pretty neat. The game has two players capturing various cities across Europe, with the goal of eventually spotting and striking the other spy down. It may be simple-looking, but after... | Read more »
Two Spies is a turn-based game for iOS t...
There aren't too many games that feature pass and play multiplayer and there are even less where you can only play against people you know, even when playing online. But Two Spies does both of those things and you can get it for iOS right now. [... | Read more »
Solve your way through new low-poly puzz...
The best escape-the-room games don’t just test your creative problem-solving skills – they look great, too. Released in October this year by Antler (the developer of the succesful VR puzzle SVRVIVE: The Deus Helix), Krystopia offers everything you... | Read more »
Get ready for an epic adventure with Pea...
Following a hugely successful pre-registration campaign, Pearl Abyss' much-hyped MMORPG, Black Desert Mobile, has finally arrived for iOS and Android. With some of the most impressive visuals on mobile, a vast open world to explore, an in-depth... | Read more »
Elder Scrolls: Blades has ditched chest...
Elder Scrolls: Blades started out as one of the most hyped mobile games of 2019, boasting some impressive visuals and no shortage of promise. Our hopes were somewhat dashed when it eventually launched and we all became privy to its mishandled... | Read more »
Hands-On with the Pocket City December U...
At the end of last month, Codebrew Games announced an update coming to their popular city-builder, Pocket City some time this month. In this update is the promise of expanding your city out into other regions, enacting policies, and more. The full... | Read more »
Black Desert Mobile is available for pre...
Pearl Abyss' stunning open-world MMORPG, Black Desert Mobile, is set to launch for iOS and Android on December 11th at 12 AM PST (8 AM UTC). However, those looking to get in early and test out the in-depth character customisation will be able to... | Read more »
Extraordinary Ones, NetEase's innov...
NetEase's inventive 5v5 anime MOBA, Extraordinary Ones, has now opened for pre-registration ahead of its global launch in early 2020. The game seems to have received a fairly warm reception from fans after its soft-launch earlier in the year,... | Read more »

Price Scanner via MacPrices.net

Apple Watch Series 3 models on sale at Amazon...
Amazon has Apple Watch Series 3 GPS models on sale for $20 off MSRP, starting at only $179. Their prices are the lowest available for these models from any Apple reseller. Choose Amazon as the seller... Read more
Sunday AirPods Sale: Amazon drops prices to a...
Amazon has new 2019 Apple AirPods on sale today ranging up to $30 off MSRP, starting at $139. Shipping is free: – AirPods Pro: $249 $0 off MSRP – AirPods with Wireless Charging Case: $168.95 $30 off... Read more
Holiday 2019 sale: 11″ iPad Pros for up to $2...
Amazon has new Apple 11″ iPad Pros in stock today and on sale for up to $200 off Apple’s MSRP as part of their Holiday 2019 sale. These are the same iPad Pros sold by Apple in its retail and online... Read more
B&H has 12.9″ WiFi iPad Pros on sale for...
B&H Photo has new 12.9″ WiFi iPad Pros on sale for up to $150 off Apple’s MSRP as part of their Holiday 2019 sale. Overnight shipping is free to many addresses in the US: – 12.9″ 64GB WiFi iPad... Read more
Find the best Holiday 2019 prices on Apple’s...
Our Apple award-winning price trackers are the best place to look for the best deals and lowest prices on Apple gear this 2019 Holiday shopping season. Scan our price trackers for the latest... Read more
13″ 2.4GHz/256GB Silver MacBook Pro on sale f...
Amazon has the Silver 13″ 2.4GHz/256GB 4-Core Touch Bar MacBook Pro on sale for $1499.99 shipped. Their price is $300 off Apple’s MSRP, and it’s the lowest price currently available for a 13″ 2.4GHz... Read more
Sams Club one day sales event December 14th:...
Through midnight Saturday night (December 14th), Sams Club online has several Apple Watch Series 5 models on sale for $40 off MSRP as part of their One Day sales event. Choose free shipping or free... Read more
Total Wireless offers iPhone 6S models for as...
Total Wireless has Apple 32GB iPhone 6S models available starting at $99: – 32GB iPhone 6S: $99.99 – 32GB iPhone 6S Plus: $149.99 A no-contract Total Wireless prepaid plan is required with your... Read more
Get a 4 or 6-core Mac Mini for up to $170 off...
B&H Photo has 4-Core and 6-Core Mac minis on sale for up to $170 off Apple’s standard MSRP as part of their Holiday 2019 sale. Overnight shipping is free to many US addresses: – 3.6GHz Quad-Core... Read more
Amazon restocks base 13″ 1.4GHz MacBook Pro f...
Amazon has restocked the base 13″ 1.4GHz/128GB Space Gray MacBook Pro for $1099.99 shipped. Their price is $200 off Apple’s MSRP, and it’s the cheapest price available for a new MacBook Pro. Amazon... Read more

Jobs Board

*Apple* Mobility Sales Professional - Best B...
**750138BR** **Job Title:** Apple Mobility Sales Professional **Job Category:** Store Associates **Store NUmber or Department:** 000471-Mt Vernon-Store **Job Read more
*Apple* Engineering Specialist (ITC ) - Gene...
…Suitability clearance, per contract requirements. Currently, we are seeking an Apple Engineering Specialist in Washington, DC The responsibilities for candidates in Read more
Senior *Apple* Endpoint Engineer - Leidos (...
…Medicaid Service (CMS) End User environment. Perform specific duties as an Apple Endpoint Engineer in support of the infrastructure operations, hardware, software Read more
Perioperative - RN - ( *Apple* Hill Surgical...
Perioperative - RN - ( Apple Hill Surgical Center) Tracking Code 59281 Job Description Monday - Friday - Part Time - Days Possible Saturdays General Summary: Under Read more
Lead DevOps Engineer - *Apple* - Theorem, L...
Job Summary Apple is looking for a seasoned Lead DevOps Engineer that can lead multiple projects and teams while delivering high quality and performant solutions in Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.