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

Maya 2016 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
RapidWeaver 6.2.3 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
MacFamilyTree 7.5.2 - Create and explore...
MacFamilyTree gives genealogy a facelift: it's modern, interactive, incredibly fast, and easy to use. We're convinced that generations of chroniclers would have loved to trade in their genealogy... Read more
Paragraphs 1.0.1 - Writing tool just for...
Paragraphs is an app just for writers. It was built for one thing and one thing only: writing. It gives you everything you need to create brilliant prose and does away with the rest. Everything in... Read more
BlueStacks App Player 0.9.21 - Run Andro...
BlueStacks App Player lets you run your Android apps fast and fullscreen on your Mac. Version 0.9.21: Note: Now requires OS X 10.8 or later running on a 64-bit Intel processor. Initial stable... Read more
Tweetbot 2.0.2 - Popular Twitter client....
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
Apple iBooks Author 2.3 - Create and pub...
Apple iBooks Author helps you create and publish amazing Multi-Touch books for iPad. Now anyone can create stunning iBooks textbooks, cookbooks, history books, picture books, and more for iPad. All... Read more
NeoOffice 2014.12 - Mac-tailored, OpenOf...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
Apple iTunes 12.2 - Play Apple Music...
Apple iTunes lets you organize and stream Apple Music, download and watch video and listen to Podcasts. It can automatically download new music, app, and book purchases across all your devices and... Read more
Apple Security Update 2015-005 - For OS...
Apple Security Update 2015-005 is recommended for all users and improves the security of OS X. For detailed information about the security content of this update, please visit: http://support.apple.... Read more

Rage of Bahamut is Giving Almost All of...
The App Store isn't what it used to be back in 2012, so it's not unexpected to see some games changing their structures with the times. Now we can add Rage of Bahamut to that list with the recent announcement that the game is severely cutting back... | Read more »
Adventures of Pip (Games)
Adventures of Pip 1.0 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ** ONE WEEK ONLY — 66% OFF! *** “Adventures of Pip is a delightful little platformer full of charm, challenge and impeccable... | Read more »
Divide By Sheep - Tips, Tricks, and Stre...
Who would have thought splitting up sheep could be so involved? Anyone who’s played Divide by Sheep, that’s who! While we’re not about to give you complete solutions to everything (because that’s just cheating), we will happily give you some... | Read more »
NaturalMotion and Zynga Have Started Tea...
An official sequel to 2012's CSR Racing is officially on the way, with Zynga and NaturalMotion releasing a short teaser trailer to get everyone excited. Well, as excited as one can get from a trailer with no gameplay footage, anyway. [Read more] | Read more »
Grab a Friend and Pick up Overkill 3, Be...
Overkill 3 is a pretty enjoyable third-person shooter that was sort of begging for some online multiplayer. Fortunately the begging can stop, because its newest update has added an online co-op mode. [Read more] | Read more »
Scanner Pro's Newest Update Adds Au...
Scanner Pro is one of the most popular document scanning apps on iOS, thanks in no small part to its near-constant updates, I'm sure. Now we're up to update number six, and it adds some pretty handy new features. [Read more] | Read more »
Heroki (Games)
Heroki 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: CLEAR THE SKIES FOR A NEW HERO!The peaceful sky village of Levantia is in danger! The dastardly Dr. N. Forchin and his accomplice,... | Read more »
Wars of the Roses (Games)
Wars of the Roses 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
TapMon Battle (Games)
TapMon Battle 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: It's time to battle!Tap! Tap! Tap! Try tap a egg to hatch a Tapmon!Do a battle with another tapmons using your hatched tapmons! *... | Read more »
Alchemic Dungeons (Games)
Alchemic Dungeons 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: ### Release Event! ### 2.99$->0.99$ for limited time! ### Roguelike Role Playing Game! ### Alchemic Dungeons is roguelike... | Read more »

Price Scanner via MacPrices.net

15-inch 2.5GHz Retina MacBook Pro on sale for...
Amazon.com has the 15″ 2.5GHz Retina MacBook Pro on sale for $2274 including free shipping. Their price is $225 off MSRP, and it’s the lowest price available for this model. Read more
Logo Pop Free Vector Logo Design App For OS X...
128bit Technologies has released of Logo Pop Free 1.2 for Mac OS X, a vector based, full-fledged, logo design app available exclusively on the Mac App Store for the agreeable price of absolutely free... Read more
21-inch 1.4GHz iMac on sale for $999, save $1...
B&H Photo has new 21″ 1.4GHz iMac on sale for $999 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Best Buy has the 21″ 1.4GHz iMac on sale for $999.99 on their... Read more
16GB iPad mini 3 on sale for $339, save $60
B&H Photo has the 16GB iPad mini 3 WiFi on sale for $339 including free shipping plus NY tax only. Their price is $60 off MSRP. Read more
Save up to $40 on iPad Air 2, NY tax only, fr...
B&H Photo has iPad Air 2s on sale for up to $40 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $489 $10 off - 64GB iPad Air 2 WiFi: $559 $40 off - 128GB iPad Air... Read more
Apple Releases OS X 10.10.4 With WIFi Fix, iO...
On Tuesday, Apple released final versions of OS X 10.10.4 and iOS 8.4, as well as updates for the Safari browser for OS X Yosemite, Mavericks, and Mountain Lion. The OS X 10.10.4 update focuses on... Read more
Dual-Band High-Gain Antennas for Home Wi-Fi N...
Linksys has announced what it claims are the first dual-band, omni-directional high-gain antennas for the consumer market. The new Linksys high-gain antennas available in a 2- and 4-pack (WRT004ANT... Read more
Apple refurbished 2014 15-inch Retina MacBook...
The Apple Store has Apple Certified Refurbished 2014 15″ 2.2GHz Retina MacBook Pros available for $1609, $390 off original MSRP. Apple’s one-year warranty is included, and shipping is free. They have... Read more
Clearance 2014 MacBook Airs available for up...
Adorama has 2014 MacBook Airs on sale for up to $301 off original MSRP including NY + NJ sales tax and free shipping: - 11″ 256GB MacBook Air: $798 $301 off original MSRP - 13″ 128GB MacBook Air: $... Read more
5K iMacs on sale for $100 off MSRP, free ship...
B&H Photo has the new 27″ 3.3GHz 5K iMac on sale for $1899.99 including free shipping plus NY tax only. Their price is $100 off MSRP. They have the 27″ 3.5GHz 5K iMac on sale for $2199, also $100... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Fulfillment Operations Execution Ana...
**Job Summary** The AMR Apple Fulfillment Operations Team is seeking a talented team player to drive the Apple Online Store (AOS) fulfillment performance to ensure a Read more
Localization Producer - *Apple* HR and Reta...
…project manager to support the Retail Globalization team. You will participate in Apple exponential inte ational growth and drive global project initiatives for the Read more
*Apple* Online Store UAT Lead - Apple (Unite...
**Job Summary** The Apple Online Store is a fast paced and ever evolving business environment. A UAT lead in this organization is able to have a direct impact on one of Read more
Senior Payments Security Manager - *Apple*...
**Job Summary** Apple , Inc. is looking for a highly motivated, innovative and hands-on senior payments security manager to join the Apple Pay security team. You will Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.