Difference between pages "Awk by Example, Part 1" and "Repository Configuration"

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(Undo revision 9202 by Duncan.britton (talk))
 
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{{Article
+
{{Warning|This article is a work-in-progress referring to a future Portage version. It does not apply to the current Funtoo Portage version. Please do not update your configuration yet.}}
|Author=Drobbins
+
|Next in Series=Awk by Example, Part 2
+
}}
+
  
== An intro to the great language with the strange name ==
+
Starting with Portage-2.3.8, a switch to a new repository configuration framework is complete and users may want to update their configuration files. This document aims to describe the goals for the new framework and how to use it.
  
=== In defense of awk ===
+
== Multiple repository layout ==
In this series of articles, I'm going to turn you into a proficient awk coder. I'll admit, awk doesn't have a very pretty or particularly "hip" name, and the GNU version of awk, called gawk, sounds downright weird. Those unfamiliar with the language may hear "awk" and think of a mess of code so backwards and antiquated that it's capable of driving even the most knowledgeable UNIX guru to the brink of insanity (causing him to repeatedly yelp "kill -9!" as he runs for coffee machine).
+
One of the most important changes is the switch from the old ''overlay'' layout to a new cleaner ''repository'' system. The new layout is more flexible and more predictable. For example, repositories can now use resources (eclasses, for example) provided by other repositories.
  
Sure, awk doesn't have a great name. But it is a great language. Awk is geared toward text processing and report generation, yet features many well-designed features that allow for serious programming. And, unlike some languages, awk's syntax is familiar, and borrows some of the best parts of languages like C, python, and bash (although, technically, awk was created before both python and bash). Awk is one of those languages that, once learned, will become a key part of your strategic coding arsenal.
+
The old layout was based on the concept of one ''main tree'' and optionally a number of overlays. The main tree provided base system ebuilds, eclasses, profiles, while overlays mostly were able to provide their own ebuilds. The ebuild provided by overlays overrode the ebuilds in main tree to the extend of making it impossible to install the main tree version. Overlays could also provide eclasses for their own ebuilds and package.* entries that applied to all overlays and to the main tree. The Package Manager is responsible for updating the main tree, while overlays are managed externally.
  
=== The first awk ===
+
The new layout is based on the concept of one or more configurable repositories. Each repository can either be stand-alone or depend upon other repositories. The distribution provides a repository called ''funtoo'' (a drop-in replacement for Gentoo's ''gentoo'' repository). Users can install more repositories at they will, the repositories providing their own ebuilds, eclasses and profiles as necessary and/or using them from other repositories. Users can explicitly choose the repository they want to install packages from. The Package Manager can update all repositories.
Let's go ahead and start playing around with awk to see how it works. At the command line, enter the following command:
+
  
<console>$##i## awk '{ print }' /etc/passwd</console>
+
== Portage configuration ==
 +
=== New repository layout ===
 +
The repository configuration should be stored in <code>/etc/portage/repos.conf</code>. It can be either a single file or a directory containing one or more ''.conf'' files.
  
You should see the contents of your /etc/passwd file appear before your eyes. Now, for an explanation of what awk did. When we called awk, we specified /etc/passwd as our input file. When we executed awk, it evaluated the print command for each line in /etc/passwd, in order. All output is sent to stdout, and we get a result identical to catting /etc/passwd.
+
The default configuration is installed as <code>/usr/share/portage/config/repos.conf</code>. This file is internal configuration file installed with portage ebuild and should '''not''' be modified. Instead, the configuration in <code>/etc/portage/repos.conf</code> can override the defaults specified there.
  
Now, for an explanation of the { print } code block. In awk, curly braces are used to group blocks of code together, similar to C. Inside our block of code, we have a single print command. In awk, when a print command appears by itself, the full contents of the current line are printed.
+
The configuration uses format similar to Windows .ini files. Each section heading (repository name in square brackets) signifies a single repository, followed by one or more key-value option pairs. For example, the following file copies default configuration for Funtoo repository:
  
Here is another awk example that does exactly the same thing:
+
{{file|name=/etc/portage/repos.conf/funtoo.conf|desc=Example configuration override for Funtoo repository to move it to non-standard location|body=
 +
[funtoo]
 +
# moved to non-standard location!
 +
location = /var/db/repos/funtoo
 +
sync-type = git
 +
sync-uri = git://github.com/funtoo/ports-2015.git
 +
auto-sync = yes
 +
}}
  
<console>$##i## awk '{ print $0 }' /etc/passwd</console>
+
The most useful repository configuration options are listed below:
 +
;location: ''Obligatory.'' Specifies the directory where repository is/will be stored. If Portage knows how to sync the repository and the location does not exist, it will be created on next ''emerge --sync''. Otherwise, the directory must exist.
 +
;priority: Specifies the priority used for ordering ebuilds from different repositories. If two repositories provide an ebuild with matching versions, the repository with higher priority will be used.
 +
;auto-sync: Specifies whether ''emerge --sync'' should update the repository. Defaults to ''yes'' if ''sync-type'' is specified, ''no'' otherwise.
 +
;sync-depth: Specifies ''--depth'' for git clone. Used only on initial sync. Defaults to 1. Can be set to 0 to force full clone (not pass ''--depth'' at all).
 +
;sync-type: Specifies syncing/update method. Can be one of: ''cvs'', ''git'', ''rsync'', ''svn''.
 +
;sync-umask: Specifies the umask used when updating/syncing the repository.
 +
;sync-uri: Specifies remote URI from which the repository will be cloned/synced. Can use any syntax valid for a particular syncing method.
 +
;sync-user: Specifies the user[:group] used to update/sync the repository. If ''FEATURES=usersync'' is used, defaults to the credentials of directory owner.
  
In awk, the $0 variable represents the entire current line, so print and print $0 do exactly the same thing. If you'd like, you can create an awk program that will output data totally unrelated to the input data. Here's an example:
+
Additionally a <code>[DEFAULT]</code> section may be specified. Options in this section are used as defaults for all repositories.
  
<console>$##i## awk '{ print "" }' /etc/passwd</console>
+
=== Migrating existing configurations ===
 +
The new configuration format provides replacement for existing configuration done through <code>/etc/portage/make.conf</code> and environment variables. While the variables are still supported for backwards compatibility, users are recommended to move to the new configuration scheme. Funtoo portage ebuild is planned to  make the migration unattended (repos.conf installed automatically to ease the config steps) with the following file:
 +
 +
{{file|name=/etc/portage/repos.conf/funtoo.conf|body=
 +
[funtoo]
 +
location = /usr/portage
 +
sync-type = git
 +
sync-uri = git://github.com/funtoo/ports-2015.git
 +
auto-sync = yes
 +
}}
 +
The following replacements are provided for existing variables:
 +
;PORTDIR: Used to specify main tree location. Replaced by ''location'' key in the section corresponding to the default repository (<code>[funtoo]</code> by default).
 +
;PORTDIR_OVERLAY: Used to specify locations of overlays. Each of the paths needs to be replaced with a separate repository section, with the path placed in ''location'' key. Additionally, ''priority'' may be used to force specific ordering of ebuild overrides.
 +
;SYNC: Used to specify URI for syncing the main repository, also implied a protocol for doing that. Replaced by the ''sync-uri'' and ''sync-type'' keys in the default repository section.
 +
;SYNC_UMASK: Used to specify umask for syncing repositories. Replaced by ''sync-umask'' key in repository configuration. Can be specified in <code>[DEFAULT]</code> section to apply to all repositories.
 +
;SYNC_USER: Used to specify user credentials for syncing repositories. Replaced by ''sync-user'' key in repository configuration. Can be specified in <code>[DEFAULT]</code> section to apply to all repositories.
  
Whenever you pass the "" string to the print command, it prints a blank line. If you test this script, you'll find that awk outputs one blank line for every line in your /etc/passwd file. Again, this is because awk executes your script for every line in the input file. Here's another example:
+
{{file|name=/etc/portage/make.conf|desc=Example old make.conf file|body=
 +
# user changed PORTDIR location
 +
PORTDIR="/var/db/repos/funtoo"
 +
PORTDIR_OVERLAY="/var/db/repos/foo /var/db/repos/bar"
  
<console>$##i## awk '{ print "hiya" }' /etc/passwd</console>
+
SYNC="git://github.com/funtoo/ports-2015.git"
 +
SYNC_USER="oleg"
 +
SYNC_UMASK="022"
 +
}}
  
Running this script will fill your screen with hiya's. :)
+
{{file|name=/etc/portage/repos.conf|desc=Replacement repos.conf file|body=
 +
[DEFAULT]
 +
sync-user = oleg
 +
sync-umask = 022
  
=== Multiple fields ===
+
[funtoo]
Awk is really good at handling text that has been broken into multiple logical fields, and allows you to effortlessly reference each individual field from inside your awk script. The following script will print out a list of all user accounts on your system:
+
location = /var/db/repos/funtoo
 +
sync-type = git
 +
sync-uri = git://github.com/funtoo/ports-2015.git
  
<console>$##i## awk -F":" '{ print $1 }' /etc/passwd</console>
+
[foo]
 +
location = /var/db/repos/foo
 +
priority = 1
  
Above, when we called awk, we use the -F option to specify ":" as the field separator. When awk processes the print $1 command, it will print out the first field that appears on each line in the input file. Here's another example:
+
[bar]
 +
location = /var/db/repos/bar
 +
priority = 2
 +
}}
  
<console>$##i## awk -F":" '{ print $1 $3 }' /etc/passwd</console>  
+
The <code>repos.conf</code> configuration can be further extended with ''sync-type'' and ''sync-uri'' for overlays to get ''emerge --sync'' updating them automatically.
  
Here's an excerpt of the output from this script:
+
let's see a real example of tree and overlays added.  
<pre>
+
{{file|name=/etc/portage/repos.conf|desc=Replacement repos.conf file|body=
halt7
+
operator11
+
root0
+
shutdown6
+
sync5
+
bin1
+
....etc.  
+
</pre>
+
As you can see, awk prints out the first and third fields of the /etc/passwd file, which happen to be the username and uid fields respectively. Now, while the script did work, it's not perfect -- there aren't any spaces between the two output fields! If you're used to programming in bash or python, you may have expected the print $1 $3 command to insert a space between the two fields. However, when two strings appear next to each other in an awk program, awk concatenates them without adding an intermediate space. The following command will insert a space between both fields:
+
  
<console>$##i## awk -F":" '{ print $1 " " $3 }' /etc/passwd</console>
+
[gentoo]
 
+
location = /usr/portage
When you call print this way, it'll concatenate $1, " ", and $3, creating readable output. Of course, we can also insert some text labels if needed:
+
sync-type = git
 
+
sync-uri = git://github.com/funtoo/ports-2012.git
<console>$##i## awk -F":" '{ print "username: " $1 "\t\tuid:" $3 }' /etc/passwd</console>
+
 
+
[funtoo-overlay]
This will cause the output to be:
+
location = /root/git/funtoo-overlay
<pre>
+
   
username: halt    uid:7
+
[funtoo-gnome]
username: operator uid:11
+
location = /root/git/funtoo-gnome-overlay
username: root    uid:0
+
}}
username: shutdown uid:6
+
funtoo-overlay and funtoo-gnome-overlay are an overlays added on top of regular portage tree.
username: sync     uid:5
+
[[Category:Portage]]
username: bin      uid:1
+
....etc.
+
</pre>
+
 
+
=== External Scripts ===
+
Passing your scripts to awk as a command line argument can be very handy for small one-liners, but when it comes to complex, multi-line programs, you'll definitely want to compose your script in an external file. Awk can then be told to source this script file by passing it the -f option:
+
 
+
<console>$##i## awk -f myscript.awk myfile.in </console>
+
 
+
Putting your scripts in their own text files also allows you to take advantage of additional awk features. For example, this multi-line script does the same thing as one of our earlier one-liners, printing out the first field of each line in /etc/passwd:
+
<pre>
+
BEGIN {
+
        FS=":"
+
}
+
{ print $1 }
+
</pre>
+
The difference between these two methods has to do with how we set the field separator. In this script, the field separator is specified within the code itself (by setting the FS variable), while our previous example set FS by passing the -F":" option to awk on the command line. It's generally best to set the field separator inside the script itself, simply because it means you have one less command line argument to remember to type. We'll cover the FS variable in more detail later in this article.
+
 
+
It is also possible to make the script directly executable, by placing a "#!/usr/bin/awk -f" at the top of the file, as follows:
+
<pre>
+
#!/usr/bin/awk -f
+
BEGIN {
+
FS=":"
+
}
+
{ print $1 }
+
</pre>
+
Next, the script must be made executable by setting the script file's execute bit:
+
 
+
<console>$##i## chmod +x myscript.awk</console>
+
 
+
Now, you should be able to execute the script as follows:
+
 
+
<console>$##i## ./myscript.awk myfile.in</console>
+
 
+
=== The BEGIN and END blocks ===
+
Normally, awk executes each block of your script's code once for each input line. However, there are many programming situations where you may need to execute initialization code before awk begins processing the text from the input file. For such situations, awk allows you to define a BEGIN block. We used a BEGIN block in the previous example. Because the BEGIN block is evaluated before awk starts processing the input file, it's an excellent place to initialize the FS (field separator) variable, print a heading, or initialize other global variables that you'll reference later in the program.
+
 
+
Awk also provides another special block, called the END block. Awk executes this block after all lines in the input file have been processed. Typically, the END block is used to perform final calculations or print summaries that should appear at the end of the output stream.
+
 
+
=== Regular expressions and blocks ===
+
Awk allows the use of regular expressions to selectively execute an individual block of code, depending on whether or not the regular expression matches the current line. Here's an example script that outputs only those lines that contain the character sequence foo:
+
 
+
<pre>/foo/ { print }</pre>
+
 
+
Of course, you can use more complicated regular expressions. Here's a script that will print only lines that contain a floating point number:
+
 
+
<pre>/[0-9]+\.[0-9]*/ { print }</pre>
+
 
+
=== Expressions and blocks ===
+
There are many other ways to selectively execute a block of code. We can place any kind of boolean expression before a code block to control when a particular block is executed. Awk will execute a code block only if the preceding boolean expression evaluates to true. The following example script will output the third field of all lines that have a first field equal to fred. If the first field of the current line is not equal to fred, awk will continue processing the file and will not execute the print statement for the current line:
+
 
+
<pre>$1 == "fred" { print $3 }</pre>
+
 
+
Awk offers a full selection of comparison operators, including the usual "==", "<", ">", "<=", ">=", and "!=". In addition, awk provides the "~" and "!~" operators, which mean "matches" and "does not match". They're used by specifying a variable on the left side of the operator, and a regular expression on the right side. Here's an example that will print only the third field on the line if the fifth field on the same line contains the character sequence root:
+
 
+
<pre>$5 ~ /root/ { print $3 }</pre>
+
 
+
=== Conditional statements ===
+
Awk also offers very nice C-like if statements. If you'd like, you could rewrite the previous script using an if statement:
+
<pre>
+
{
+
    if ( $5 ~ /root/ ) {
+
        print $3
+
    }
+
}
+
</pre>
+
Both scripts function identically. In the first example, the boolean expression is placed outside the block, while in the second example, the block is executed for every input line, and we selectively perform the print command by using an if statement. Both methods are available, and you can choose the one that best meshes with the other parts of your script.
+
 
+
Here's a more complicated example of an awk if statement. As you can see, even with complex, nested conditionals, if statements look identical to their C counterparts:
+
<pre>
+
{
+
    if ( $1 == "foo" ) {
+
        if ( $2 == "foo" ) {
+
            print "uno"
+
        } else {
+
            print "one"
+
        }
+
    } else if ($1 == "bar" ) {
+
        print "two"
+
    } else {
+
        print "three"
+
    }
+
}
+
</pre>
+
Using if statements, we can also transform this code:
+
<pre>
+
! /matchme/ { print $1 $3 $4 }
+
</pre>
+
to this:
+
<pre>
+
{
+
    if ( $0 !~ /matchme/ ) {
+
        print $1 $3 $4
+
    }
+
}
+
</pre>
+
Both scripts will output only those lines that don't contain a matchme character sequence. Again, you can choose the method that works best for your code. They both do the same thing.
+
 
+
Awk also allows the use of boolean operators "||" (for "logical or") and "&&"(for "logical and") to allow the creation of more complex boolean expressions:
+
<pre>
+
( $1 == "foo" ) && ( $2 == "bar" ) { print }
+
</pre>
+
This example will print only those lines where field one equals foo and field two equals bar.
+
 
+
=== Numeric variables! ===
+
So far, we've either printed strings, the entire line, or specific fields. However, awk also allows us to perform both integer and floating point math. Using mathematical expressions, it's very easy to write a script that counts the number of blank lines in a file. Here's one that does just that:
+
<pre>
+
BEGIN { x=0 }
+
/^$/ { x=x+1 }
+
END  { print "I found " x " blank lines. :)" }
+
</pre>
+
In the BEGIN block, we initialize our integer variable x to zero. Then, each time awk encounters a blank line, awk will execute the x=x+1 statement, incrementing x. After all the lines have been processed, the END block will execute, and awk will print out a final summary, specifying the number of blank lines it found.
+
 
+
=== Stringy variables ===
+
One of the neat things about awk variables is that they are "simple and stringy." I consider awk variables "stringy" because all awk variables are stored internally as strings. At the same time, awk variables are "simple" because you can perform mathematical operations on a variable, and as long as it contains a valid numeric string, awk automatically takes care of the string-to-number conversion steps. To see what I mean, check out this example:
+
<pre>
+
x="1.01"
+
# We just set x to contain the *string* "1.01"
+
x=x+1
+
# We just added one to a *string*
+
print x
+
# Incidentally, these are comments :)
+
</pre>
+
Awk will output:
+
<pre>
+
2.01
+
</pre>
+
Interesting! Although we assigned the string value 1.01 to the variable x, we were still able to add one to it. We wouldn't be able to do this in bash or python. First of all, bash doesn't support floating point arithmetic. And, while bash has "stringy" variables, they aren't "simple"; to perform any mathematical operations, bash requires that we enclose our math in an ugly $( ) construct. If we were using python, we would have to explicitly convert our 1.01 string to a floating point value before performing any arithmetic on it. While this isn't difficult, it's still an additional step. With awk, it's all automatic, and that makes our code nice and clean. If we wanted to square and add one to the first field in each input line, we would use this script:
+
<pre>
+
{ print ($1^2)+1 }
+
</pre>
+
If you do a little experimenting, you'll find that if a particular variable doesn't contain a valid number, awk will treat that variable as a numerical zero when it evaluates your mathematical expression.
+
 
+
=== Lots of operators ===
+
Another nice thing about awk is its full complement of mathematical operators. In addition to standard addition, subtraction, multiplication, and division, awk allows us to use the previously demonstrated exponent operator "^", the modulo (remainder) operator "%", and a bunch of other handy assignment operators borrowed from C.
+
 
+
These include pre- and post-increment/decrement ( i++, --foo ), add/sub/mult/div assign operators ( a+=3, b*=2, c/=2.2, d-=6.2 ). But that's not all -- we also get handy modulo/exponent assign ops as well ( a^=2, b%=4 ).
+
 
+
=== Field separators ===
+
Awk has its own complement of special variables. Some of them allow you to fine-tune how awk functions, while others can be read to glean valuable information about the input. We've already touched on one of these special variables, FS. As mentioned earlier, this variable allows you to set the character sequence that awk expects to find between fields. When we were using /etc/passwd as input, FS was set to ":". While this did the trick, FS allows us even more flexibility.
+
 
+
The FS value is not limited to a single character; it can also be set to a regular expression, specifying a character pattern of any length. If you're processing fields separated by one or more tabs, you'll want to set FS like so:
+
<pre>
+
FS="\t+"
+
</pre>
+
Above, we use the special "+" regular expression character, which means "one or more of the previous character".
+
 
+
If your fields are separated by whitespace (one or more spaces or tabs), you may be tempted to set FS to the following regular expression:
+
<pre>
+
FS="[[:space:]]+"
+
</pre>
+
While this assignment will do the trick, it's not necessary. Why? Because by default, FS is set to a single space character, which awk interprets to mean "one or more spaces or tabs." In this particular example, the default FS setting was exactly what you wanted in the first place!
+
 
+
Complex regular expressions are no problem. Even if your records are separated by the word "foo," followed by three digits, the following regular expression will allow your data to be parsed properly:
+
<pre>
+
FS="foo[0-9][0-9][0-9]"
+
</pre>
+
 
+
=== Number of fields ===
+
The next two variables we're going to cover are not normally intended to be written to, but are normally read and used to gain useful information about the input. The first is the NF variable, also called the "number of fields" variable. Awk will automatically set this variable to the number of fields in the current record. You can use the NF variable to display only certain input lines:
+
<pre>
+
NF == 3 { print "this particular record has three fields: " $0 }
+
</pre>
+
Of course, you can also use the NF variable in conditional statements, as follows:
+
<pre>
+
{
+
    if ( NF > 2 ) {
+
        print $1 " " $2 ":" $3
+
    }
+
}
+
</pre>
+
 
+
=== Record number ===
+
The record number (NR) is another handy variable. It will always contain the number of the current record (awk counts the first record as record number 1). Up until now, we've been dealing with input files that contain one record per line. For these situations, NR will also tell you the current line number. However, when we start to process multi-line records later in the series, this will no longer be the case, so be careful! NR can be used like the NF variable to print only certain lines of the input:
+
<pre>
+
(NR < 10 ) || (NR > 100) { print "We are on record number 1-9 or 101+" }
+
</pre>
+
<pre>
+
{
+
    #skip header
+
    if ( NR > 10 ) {
+
        print "ok, now for the real information!"
+
    }
+
}
+
</pre>
+
Awk provides additional variables that can be used for a variety of purposes. We'll cover more of these variables in later articles.
+
 
+
We've come to the end of our initial exploration of awk. As the series continues, I'll demonstrate more advanced awk functionality, and we'll end the series with a real-world awk application.
+
 
+
== Resources ==
+
 
+
* Read Daniel's other awk articles on Funtoo: Awk By Example, [[Awk by example, Part2 |Part 2]] and [[Awk by example, Part3 |Part 3]].
+
* If you'd like a good old-fashioned book, [http://www.oreilly.com/catalog/sed2/ O'Reilly's sed & awk, 2nd Edition] is a wonderful choice.
+
* Be sure to check out the [http://www.faqs.org/faqs/computer-lang/awk/faq/ comp.lang.awk FAQ]. It also contains lots of additional awk links.
+
* Patrick Hartigan's [http://sparky.rice.edu/~hartigan/awk.html awk tutorial] is packed with handy awk scripts.
+
* [http://www.tasoft.com/tawk.html Thompson's TAWK Compiler] compiles awk scripts into fast binary executables. Versions are available for Windows, OS/2, DOS, and UNIX.
+
* [http://www.gnu.org/software/gawk/manual/gawk.html The GNU Awk User's Guide] is available for online reference.
+
* [http://www.folkstalk.com/2011/12/good-examples-of-awk-command-in-unix.html Awk Command] daily useful examples.
+
[[Category:Linux Core Concepts]]
+
[[Category:Articles]]
+
{{ArticleFooter}}
+

Revision as of 04:52, February 27, 2015

Warning

This article is a work-in-progress referring to a future Portage version. It does not apply to the current Funtoo Portage version. Please do not update your configuration yet.

Starting with Portage-2.3.8, a switch to a new repository configuration framework is complete and users may want to update their configuration files. This document aims to describe the goals for the new framework and how to use it.

Multiple repository layout

One of the most important changes is the switch from the old overlay layout to a new cleaner repository system. The new layout is more flexible and more predictable. For example, repositories can now use resources (eclasses, for example) provided by other repositories.

The old layout was based on the concept of one main tree and optionally a number of overlays. The main tree provided base system ebuilds, eclasses, profiles, while overlays mostly were able to provide their own ebuilds. The ebuild provided by overlays overrode the ebuilds in main tree to the extend of making it impossible to install the main tree version. Overlays could also provide eclasses for their own ebuilds and package.* entries that applied to all overlays and to the main tree. The Package Manager is responsible for updating the main tree, while overlays are managed externally.

The new layout is based on the concept of one or more configurable repositories. Each repository can either be stand-alone or depend upon other repositories. The distribution provides a repository called funtoo (a drop-in replacement for Gentoo's gentoo repository). Users can install more repositories at they will, the repositories providing their own ebuilds, eclasses and profiles as necessary and/or using them from other repositories. Users can explicitly choose the repository they want to install packages from. The Package Manager can update all repositories.

Portage configuration

New repository layout

The repository configuration should be stored in /etc/portage/repos.conf. It can be either a single file or a directory containing one or more .conf files.

The default configuration is installed as /usr/share/portage/config/repos.conf. This file is internal configuration file installed with portage ebuild and should not be modified. Instead, the configuration in /etc/portage/repos.conf can override the defaults specified there.

The configuration uses format similar to Windows .ini files. Each section heading (repository name in square brackets) signifies a single repository, followed by one or more key-value option pairs. For example, the following file copies default configuration for Funtoo repository:

/etc/portage/repos.conf/funtoo.conf - Example configuration override for Funtoo repository to move it to non-standard location
[funtoo]
# moved to non-standard location!
location = /var/db/repos/funtoo
sync-type = git
sync-uri = git://github.com/funtoo/ports-2015.git
auto-sync = yes

The most useful repository configuration options are listed below:

location
Obligatory. Specifies the directory where repository is/will be stored. If Portage knows how to sync the repository and the location does not exist, it will be created on next emerge --sync. Otherwise, the directory must exist.
priority
Specifies the priority used for ordering ebuilds from different repositories. If two repositories provide an ebuild with matching versions, the repository with higher priority will be used.
auto-sync
Specifies whether emerge --sync should update the repository. Defaults to yes if sync-type is specified, no otherwise.
sync-depth
Specifies --depth for git clone. Used only on initial sync. Defaults to 1. Can be set to 0 to force full clone (not pass --depth at all).
sync-type
Specifies syncing/update method. Can be one of: cvs, git, rsync, svn.
sync-umask
Specifies the umask used when updating/syncing the repository.
sync-uri
Specifies remote URI from which the repository will be cloned/synced. Can use any syntax valid for a particular syncing method.
sync-user
Specifies the user[:group] used to update/sync the repository. If FEATURES=usersync is used, defaults to the credentials of directory owner.

Additionally a [DEFAULT] section may be specified. Options in this section are used as defaults for all repositories.

Migrating existing configurations

The new configuration format provides replacement for existing configuration done through /etc/portage/make.conf and environment variables. While the variables are still supported for backwards compatibility, users are recommended to move to the new configuration scheme. Funtoo portage ebuild is planned to make the migration unattended (repos.conf installed automatically to ease the config steps) with the following file:

/etc/portage/repos.conf/funtoo.conf
[funtoo]
location = /usr/portage
sync-type = git
sync-uri = git://github.com/funtoo/ports-2015.git
auto-sync = yes

The following replacements are provided for existing variables:

PORTDIR
Used to specify main tree location. Replaced by location key in the section corresponding to the default repository ([funtoo] by default).
PORTDIR_OVERLAY
Used to specify locations of overlays. Each of the paths needs to be replaced with a separate repository section, with the path placed in location key. Additionally, priority may be used to force specific ordering of ebuild overrides.
SYNC
Used to specify URI for syncing the main repository, also implied a protocol for doing that. Replaced by the sync-uri and sync-type keys in the default repository section.
SYNC_UMASK
Used to specify umask for syncing repositories. Replaced by sync-umask key in repository configuration. Can be specified in [DEFAULT] section to apply to all repositories.
SYNC_USER
Used to specify user credentials for syncing repositories. Replaced by sync-user key in repository configuration. Can be specified in [DEFAULT] section to apply to all repositories.
/etc/portage/make.conf - Example old make.conf file
# user changed PORTDIR location
PORTDIR="/var/db/repos/funtoo"
PORTDIR_OVERLAY="/var/db/repos/foo /var/db/repos/bar"

SYNC="git://github.com/funtoo/ports-2015.git"
SYNC_USER="oleg"
SYNC_UMASK="022"
/etc/portage/repos.conf - Replacement repos.conf file
[DEFAULT]
sync-user = oleg
sync-umask = 022

[funtoo]
location = /var/db/repos/funtoo
sync-type = git
sync-uri = git://github.com/funtoo/ports-2015.git

[foo]
location = /var/db/repos/foo
priority = 1

[bar]
location = /var/db/repos/bar
priority = 2

The repos.conf configuration can be further extended with sync-type and sync-uri for overlays to get emerge --sync updating them automatically.

let's see a real example of tree and overlays added.

/etc/portage/repos.conf - Replacement repos.conf file
[gentoo]
location = /usr/portage
sync-type = git
sync-uri = git://github.com/funtoo/ports-2012.git
 
[funtoo-overlay]
location = /root/git/funtoo-overlay
 
[funtoo-gnome]
location = /root/git/funtoo-gnome-overlay

funtoo-overlay and funtoo-gnome-overlay are an overlays added on top of regular portage tree.