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I'm trying to learn to rename files with the command line, and after browsing around a lot of pages I finally found a command that uppercases the first letter of a file, but the problem is that I want to understand the meaning of each command. The command is:

for i in *; do new=`echo "$i" | sed -e 's/^./\U&/'`; mv "$i" "$new";done

I understand the 'for' kinda... but not the 'echo' or '`' and especially the sed command. if someone has a little patience to explain the meaning of each thing that'd be awesome! Thanks!

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4 Answers 4

for i in *; 
    do new=`echo "$i" | sed -e 's/^./\U&/'`;
    mv "$i" "$new";
done;

Would be the entire code broken down to make it easily viewable, I'll explain below.


First we have to make sure we process every file, the following line means: "Do the following as long as you can find files"

for i in *;

Then we have to load the current filname (which is stored in variable $i), replace the first letter by a capitalized version and store the new name (in variable $new).

The following command basically means: Make variable $new (do new=) Load $i and forward it ("$i" |) to a function that replaces the first letter by a capitalized one. (sed -e 's/^./\U&/';`)

The sed manual page can be found here. It's used to perform basic text transformations. What is basically does is use a regular expression (the 's/^./\U&/' part) (a way of detecting a certain pattern in a text) to detect first-characters that are uncapitalized and replace it with a capitalized version.

do new=echo "$i" | sed -e 's/^./\U&/';

After we've gotten the new name, it's time to rename the file with it's new capitalized name, the following command does that.

It says: move (same as rename) the old file foo.bar to the new Foo.bar.

mv "$i" "$new";

So we now have one file renamed and it's time to move to the next. The following concludes the for function, telling it it is now time to move on and process the next file.

done;
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4  
Nice explanation. It's worth noting that the \U in sed to uppercase the replacement is a GNU sed extension, and is likely not to work on UNIX systems where a different sed is used. –  coneslayer Jun 10 '10 at 18:00
    
this command appends a letter U infront of every file name. Which is undesired –  runrunforest Mar 22 '13 at 5:00

Welcome to Bash, where just about everything is possible if not immediately obvious.

BloodPhilia explanation is very good for your specific request. If you are interested, here are some additional web resources to learn more:

Bash:

Sed (which is the 'S'tream 'ED'itor):

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+1 for adding documentation –  BloodPhilia Jun 11 '10 at 4:35
    
@BloodPhilia: Thanks! –  drewk Jun 11 '10 at 15:48
    
Well it does add to my answer and even refers to it ;) Some people make a habit out of copying it without adding anything... Yours actually adds something to mine... I like that =) –  BloodPhilia Jun 11 '10 at 15:56

Here are a couple of other techniques in case your sed doesn't support \U:

for i in *; do first=$(echo "${i:0:1}" | tr '[:lower:]' '[:upper:]'); new=$first${i:1}; mv "$i" "$new"; done

If you have Bash 4:

for i in *; do mv "$i" "${i^}"; done
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And if you have zsh :

for i in *; do mv $i ${(C)i}; done
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That capitalization flag will capitalize the first character of all maximal alphanumeric subsequences, not just the first character (foo=bar.baz && echo ${(C)foo} gives Bar.Baz). Try ${(C)i[1]}${i[2,-1]}. –  Chris Johnsen Jun 11 '10 at 3:14
    
Too bad, that's less elegant.... Thanks though. –  Henno Jun 11 '10 at 9:52

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