# Parrot User's Manual

Last edited: October 2011

Parrot is Copyright (C) 2003-2004 Douglas Thain and Copyright (C) 2005- The University of Notre Dame.
All rights reserved.
This software is distributed under the GNU General Public License.
See the file COPYING for details.

- Douglas Thain and Miron Livny, Parrot: An Application Environment for Data-Intensive Computing, Scalable Computing: Practice and Experience, Volume 6, Number 3, Pages 9--18, 2005.

## Overview⇗

Parrot is a tool for attaching old programs to new storage systems. Parrot makes a remote storage system appear as a file system to a legacy application. Parrot does not require any special privileges, any recompiling, or any change whatsoever to existing programs. It can be used by normal users doing normal tasks. For example, an anonymous FTP service is made available to `vi` like so: `% parrot_run vi /anonftp/ftp.gnu.org/pub/README`

Parrot is particularly handy for distributed computing, because it allows your application to access remote software and data, regardless of where it is actually executing. For example, it is commonly used in the high energy physics community to obtain remote access to the CVMFS distributed software repository.

Almost any application - whether static or dynmically linked, standard or commercial, command-line or GUI - should work with Parrot. There are a few exceptions. Because Parrot relies on the Linux `ptrace` interface any program that relies on the ptrace interface cannot run under Parrot. This means Parrot cannot run a debugger, nor can it run itself recursively. In addition, Parrot cannot run setuid programs, as the operating system system considers this a security risk.

Parrot also provide a new experimental features called identity boxing. This feature allows you to securely run a visiting application within a protection domain without become root or creating a new account. Read below for more information on identity boxing.

Parrot currently runs on the Linux operating system with either Intel compatible (i386) or AMD compatible (x86_64) processors. It relies on some fairly low level details in order to implement system call trapping. Ports to other platforms and processors Linux may be possible in the future.

Like any software, Parrot is bound to have some bugs. Please post questions to our forum and bugs to our issue tracker.

## Installation⇗

Parrot is distributed as part of the Cooperative Computing Tools. To install, please read the cctools installation instructions.

## Examples⇗

To use Parrot, you simply use the `parrot` command followed by any other Unix program. For example, to run a Parrot-enabled `vi`, execute this command: `% parrot_run vi /anonftp/ftp.gnu.org/pub/README` Of course, it can be clumsy to put `parrot_run` before every command you run, so try starting a shell with Parrot already loaded: `% parrot_run bash` Now, you should be able to run any standard command using Parrot filenames. Here are some examples to get you thinking: `% cp /http/www.nd.edu/~dthain/papers/parrot-agm2003.pdf . % grep Yahoo /http/www.yahoo.com % set autolist % cat /anonftp/ftp.gnu.org/pub[Press TAB here]`

### Interactive Use of Parrot⇗

You may find it useful to have some visual indication of when Parrot is active, so we recommend that you modify your shell startup scripts to change the prompt when Parrot is enabled. Scripts may execute `parrot_run --is-running` to detect if Parrot is already running in the current session.

- If you use `tcsh`, add to your `.cshrc`: `which parrot_run > /dev/null && parrot_run --is-running > /dev/null if ($? == 0) then set prompt = " (Parrot) %n@%m%~%# " else set prompt = " %n@%m%~%# " endif`
- If you use `bash`, add to your `.bashrc`: `if parrot_run --is-running > /dev/null 2> /dev/null; then PS1="(Parrot) ${PS1}" fi`

## Protocols⇗

We have limited the examples so far to HTTP and anonymous FTP, as they are the only services we know that absolutely everyone is familiar with. There are a number of other more powerful and secure remote services that you may be less familiar with. Parrot supports them in the same form: The filename begins with the service type, then the host name, then the file name. Here are all the currently supported services:

example path | remote service | more info
/http/www.somewhere.com/index.html | Hypertext Transfer Protocol | included
/grow/www.somewhere.com/index.html | GROW - Global Read-Only Web Filesystem | included
/ftp/ftp.cs.wisc.edu/RoadMap | File Transfer Protocol | included
/anonftp/ftp.cs.wisc.edu/RoadMap | Anonymous File Transfer Protocol | included
/gsiftp/ftp.globus.org/path | Globus Security + File Transfer Protocol | more info
/irods/host:port/zone/home/user/path | iRODS | more info
/hdfs/namenode:port/path | Hadoop Distributed File System (HDFS) | more info
/xrootd/host:port/path | XRootD/Scalla Distributed Storage System (xrootd) | more info
/cvmfs/grid.cern.ch/path | CernVM-FS | more info
/chirp/target.cs.wisc.edu/path | Chirp Storage System | included + more info

The following protocols have been supported in the past, but are not currently in active use.

/nest/nest.cs.wisc.edu/path | Network Storage Technology | more info
/rfio/host.cern.ch/path | Castor Remote File I/O | more info
/dcap/dcap.cs.wisc.edu/pnfs/cs.wisc.edu/path | DCache Access Protocol | more info
/lfn/logical/path | Logical File Name - Grid File Access Library | more info
/srm/server/path | Site File Name - Grid File Access Library | more info
/guid/abc123 | Globally Unique File Name - Grid File Access Library | more info
/gfal/protocol://host//path | Grid File Access Library | more info

If a remote service is interfering with your system, e.g. you already have CVMFS mounted at /cvmfs, you can run Parrot as `% parrot_run --disable-service cvmfs sh` to disable Parrot's handling. This option can be specified multiple times.

You will notice quite quickly that not all remote I/O systems provide all of the functionality common to an ordinary file system. For example, HTTP is incapable of listing files. If you attempt to perform a directory listing on an HTTP server, Parrot will attempt to keep `ls` happy by producing a bogus directory entry: `% parrot_run ls -la /http/www.yahoo.com/ -r--r--r-- 1 dthain dthain 0 Jul 16 11:50 /http/www.yahoo.com`

A less-drastic example is found in FTP. If you attempt to perform a directory listing of an FTP server, Parrot fills in the available information -- the file names and their sizes -- but again inserts bogus information to fill the rest out: `% parrot_run ls -la /anonftp/ftp.gnu.org/pub -rwxrwxrwx 1 dthain dip 580 Oct 5 16:00 CRYPTO.README -rwxrwxrwx 1 dthain dip 166697 Oct 5 16:00 find.txt.gz drwxrwxrwx 1 dthain dip 0 Oct 5 16:00 gnu ...`

If you would like to get a better idea of the underlying behavior of Parrot, try running it with the `-d remote` option, which will display all of the remote I/O operations that it performs on a program's behalf: `% parrot_run -d remote ls -la /anonftp/ftp.gnu.org ... ftp.cs.wisc.edu <-- TYPE I ftp.cs.wisc.edu --> 200 Type set to I. ftp.cs.wisc.edu <-- PASV ftp.cs.wisc.edu --> 227 Entering Passive Mode (128,105,2,28,194,103) ftp.cs.wisc.edu <-- NLST / ftp.cs.wisc.edu --> 150 Opening BINARY mode data connection for file list. ...`

If your program is upset by the unusual semantics of such storage systems, then consider using the Chirp protocol and server:

## The Chirp Protocol and Server⇗

Although Parrot works with many different protocols, is it limited by the capabilities provided by each underlying system. (For example, HTTP does not have reliable directory listings.) Thus, we have developed a custom protocol, Chirp, which provides secure remote file access with all of the capabilities needed for running arbitrary Unix programs. Chirp is included with the distribution of Parrot, and requires no extra steps to install.

To start a Chirp server, simply do the following: `% chirp_server -d all` The `-d all` option turns on debugging, which helps you to understand how it works initially. You may remove this option once everything is working.

Suppose the Chirp server is running on `bird.cs.wisc.edu`. Using Parrot, you may access all of the Unix features of that host from elsewhere: `% parrot_run tcsh % cd /chirp/bird.cs.wisc.edu % ls -la % ...`

In general, Parrot gives better performance and usability with Chirp than with other protocols. You can read extensively about the Chirp server and protocol in the Chirp manual.

In addition, Parrot provides several custom command line tools (`parrot_getacl`, `parrot_setacl`, `parrot_lsalloc`, and `parrot_mkalloc`) that can be used to manage the access control and space allocation features of Chirp from the Unix command line.

## Name Resolution⇗

In addition to accessing remote storage, Parrot allows you to create a custom namespace for any program. All file name activity passes through the Parrot name resolver, which can transform any given filename according to a series of rules that you specify.

The simplest name resolver is the mountlist, given by the -m mountfile option. This file corresponds closely to `/etc/fstab` in Unix. A mountlist is simply a file with one mount entry given per line. The first column is the path in Parrot's namespace. The next column can be either an access control specifier or a path in the host's filesystem. If two paths are given, then the third column can contain an optional access control specifier.

For example, the GNU FTP server available at `/anonftp/ftp.cs.wisc.edu/db` can be spliced into the filesystem under `/gnu` with a mount list like this: `/gnu /anonftp/ftp.gnu.org` Instruct Parrot to use the mountlist as follows: `% parrot_run -m mountfile bash % cd /gnu % ls -la % exit`

An access control specifier restricts the operations allowed under a given path. `DENY` blocks all operations, `ENOENT` makes operations fail as if nothing was present at the path, and `LOCAL` only allows operation on the local filesystem. For more fine-grained control, an access specifier can also be a combination of the letters `r`, `w`, and `x` to allow read, write, and execute/search, respectively. For example, a mountlist could contain the following entries `/ RX /home DENY /tmp /tmp/sandbox /opt /home/fred/project RX`

Individual mount entries may be given on the command line with the -M option as follows: `% parrot_run -M /gnu=/anonftp/ftp.gnu.org bash`

The mount list may also be changed at runtime via `parrot_mount`, if parrot is run with the `--dynamic-mounts` option: `% parrot_run --dynamic-mounts bash % parrot_mount /gnu /anonftp/ftp.gnu.org/gnu RWX % ls /gnu % parrot_mount --unmount /gnu % exit` A more sophisticated way to perform name binding is with an external resolver. This is a program executed whenever Parrot needs to locate a file or directory. The program accepts a logical file name and then returns the physical location where it can be found.

Suppose that you have a database service that locates the nearest copy of a file for you. If you run the command `locate_file`, it will print out the nearest copy of a file. For example: `% locate_file /1523.data /chirp/server.nd.edu/mix/1523.data`

To connect the program `locate_file` to Parrot, simply give a mount string that specifies the program as a resolver: `% parrot_run -M /gnu=resolver:/path/to/locate_file bash`

Now, if you attempt to access files under /gnu, Parrot will execute `locate_file` and access the data stored there: `% cat /gnu/1523.data (see contents of /chirp/server.nd.edu/mix/1523.data)`

## Mount Namespaces⇗

Mount entries in Parrot are organized into hierarchical, reference counted namespaces. A fresh instance of Parrot puts all process into a single, global namespace. This means that normally, all processes running under a Parrot instance see the same view of the filesystem. More concretely, all processes share the same mount list specified by the `-m` and `-M` options. The `parrot_mount` command allows programs to edit this mount list.

Processes are also free to fork their mount namespaces with the Parrot-specific `parrot_fork_namespace` syscall. Whenever a process forks its namespace, its child will inherit a reference to its parent’s mount namespace. Mount namespace changes are visible to processes subject to the following rules. Given a process P in namespace N,

- P may add/remove mount entries in N.
- any other process with a reference to N, will see changes by P.
- suppose another process P’ forks N, so that P’ is in namespace N’, then
  - any changes to N’ are not visible in N.
  - any changes to N are visible in N’.
- when adding a mount entry to N, the redirect is resolved in the parent namespace of N.

A process' mount namespace forms a closure over the set of visible mount entries. Forking another namespace will capture all visible mount entries, and allow the process to make local changes independently from the parent namespace. Names are lexically scoped, i.e. a new mount is resolved in its enclosing scope. When Parrot starts, the global mount namespace has no parent, so mounts passed in are resolved in the host filesystem. Unless the `--dynamic-mounts` flag is passed, Parrot seals the global mount namespace before running the tracee by making a new child namespace. Thus all processes are, by default, locked into the namespace set up by the command line/environment variables. Any processes may add mount entries to its own mount namespace via `parrot_mount`, but may not remove mount entries defined in parent and sealed namespace(s). A process can make changes to its mount namespace, then seal it with `parrot_mount --disable` to prevent the process or its children from undoing those changes. If `--dynamic-mounts` is passed, the global namespace is left unsealed, so mounts/unmounts are resolved in the host filesystem. This allows a setup script to modify the command line mounts, then seal the global namespace so that it can no longer be modified. Likewise, a process can fork its namespace, add mount entries, then seal it so that all children will be locked into the current view of the system.

The `parrot_namespace` utility gives a more convenient way to create new mount namespaces rather than using the `parrot_fork_namespace` syscall directly. This utility forks the current mount namespace and performs any mounts specified on the command line. `parrot_namespace` detects whether or not it is already running under Parrot. If so, `parrot_namespace` uses Parrot-specific syscalls to make the mount changes in a newly-forked mount namespace. If not running under Parrot, `parrot_namespace` simply executes `parrot_run`.

For applications that want to nest Parrot sessions and only need to make changes to mounts, `parrot_namespace` should work as a drop-in replacement for `parrot_run`. `parrot_namespace` only supports a limited subset of the options available for `parrot_run`. By always using `parrot_namespace`, the user need not be concerned with whether Parrot is already running.

## Record Accessed Files and Environment Variables⇗

To figure out the underlying file dependencies and execution environment, Parrot allows you to record the names of all the accessed files during the execution process of one program, which is implemented as the --name-list dependencylist option, and allows you to record the environment variables of your program, which is implemented as the --env-list envlist option. When one filename is resolved by the Parrot name resolver, it is also recorded into the dependencylist file. The system call type of a file is also transferred to the name resolver and recorded into the dependencylist file. For example, all the accessed file names will be recorded into list.txt, and the environment variables will be recorded into envlist, if we run the following command:

`% parrot_run --name-list list.txt --env-list envlist ls ~`

The format of list.txt is filename|system-call-type, such as `usr/bin/ls|stat`, which means the file `/usr/bin/ls` is accessed using the `stat` system call.

## Generate a Package based on the Accessed Files⇗

After recording the accessed files of one program with the help of the --name-list parameter of `parrot_run` and the environment variables with the help of the --env-list parameter of `parrot_run`, `parrot_package_create` can generate a package containing all the accessed files and the environment variables. `parrot_package_create` shares the same --name-list and --env-list parameters with `parrot_run`. --package-path parameter is used to specify the location of package.

`% parrot_package_create --name-list namelist --env-list envlist --package-path /tmp/package`

After executing this command, one package with the path of /tmp/package will be generated. The envlist file, envlist will be put under /tmp/package with the name of env_list.

You can also add the dependencies recorded in a new namelist file into an existing package: `% parrot_package_create --name-list namelist1 --env-list envlist1 --new-env envlist1 --add /tmp/package`

After executing this command, all the new dependencies mentioned in namelist1 will be added into /tmp/package, the new envlist, envlist1, will also be added into /tmp/package with the name specified by the --new-env option.

## Repeat one Program within the Package⇗

Once a package is generated with the help of `parrot_package_create`, we can use `parrot_package_run` to repeat the program within the package. `parrot_package_run` is based on the mountlist redirection mechanism of `parrot_run`. One mountlist wll be created so that the file access request of your program can be redirected into the package. --package-path parameter specifies the paht of the package. If no command is given, a /bin/sh shell will be returned.

`% parrot_package_run --package-path /tmp/package /bin/bash`

After the execution of this command, one shell will be returned, where you can repeat your original program. After everything is done, exit `parrot_package_run`:

`% exit`

You can also directly set your command as the arguments of `parrot_package_run`. In this case, `parrot_package_run` will exit automatically after the command is finished, and you do not need to use `exit` to exit. However, your command must belong to the original command set executed inside `parrot_run` and preserved by `parrot_package_create`.

`% parrot_package_run --package-path /tmp/package ls -al`

You can also specify a different environment file to run programs inside a package with the --env-list option. `% parrot_package_run -env-list /tmp/package/envlist1 --package-path /tmp/package ls -al`

## Reflink Copy⇗

Parrot can take advantage of the reflink feature (added in coreutils 7.5) when using `cp`. To use this feature, invoke `cp` as `% cp --reflink foo bar` This works by intercepting `BTRFS_IOC_CLONE` to trigger an in-Parrot copy with no further interaction with `cp`, avoiding the overhead of moving data into the client's buffer and then immediately back to Parrot. When run in Parrot, `cp --reflink` is not restricted to files on the same BTRFS volume, and can be used for efficiently copying any regular file.

As of coreutils 8.24, `mv` will automatically attempt a reflink copy when moving files across mount points. Parrot's reflink feature allows e.g. `mv`ing a file into a tmpfs like `/tmp` with minimal overhead.

## More Efficient Copies with `parrot_cp`⇗

If you are using Parrot to copy lots of files across the network, you may see better performance using the `parrot_cp` tool. This program looks like an ordinary `cp`, but it makes use of an optimized Parrot system call that streams entire files over the network, instead of copying them block by block.

To use `parrot_cp`, simply use your shell to alias calls to `cp` with calls to `parrot_cp`:

`% parrot_run tcsh % alias cp parrot_cp % cp /tmp/mydata /chirp/server.nd.edu/joe/data % cp -rR /chirp/server.nd.edu/joe /tmp/joe`

If run outside of Parrot, `parrot_cp` will operate as an ordinary `cp` without any performance gain or loss.

## Notes on Protocols⇗

### HTTP Proxy Servers⇗

HTTP, CVMFS, and GROW can take advantage of standard HTTP proxy servers. To route requests through a single proxy server, set the HTTP_PROXY environment variable to the server name and port: `% setenv HTTP_PROXY "http://proxy.nd.edu:8080"`

Multiple proxy servers can be given, separated by a semicolon. This will cause Parrot to try each proxy in order until one succeeds. If `DIRECT` is given as the last name in the list, then Parrot will fall back on a direct connection to the target web server. For example: `% setenv HTTP_PROXY "http://proxy.nd.edu:8080;http://proxy.wisc.edu:1000;DIRECT"`

### GROW - Global Read Only Web Filesystem⇗

Although the strict HTTP protocol does not allow for correct structured directory listings, it is possible to emulate directory listings with a little help from the underlying filesystem. We call this technique GROW, a global filesystem based on the Web. GROW requires the exporter of data to run a script (`make_growfs`) that generates a complete directory listing of the data that you wish to export. This directory listing is then used to produce reliable metadata. Of course, if the data changes, the script must be run again, so GROW is only useful for data that changes infrequently.

To set up an GROW filesystem, you must run `make_growfs` on the web server machine with the name of the local storage directory as the argument. For example, suppose that the web server `my.server.com` stores pages for the URL `http://my.server.com/~fred` in the local directory `/home/fred/www`. In this case, you should run the following command:

`% make_growfs /home/fred/www` Now, others may perceive the web server as a file server under the /grow hierarchy. For example: `% parrot_run tcsh % cd /grow/my.server.com/~fred % ls -la`

In addition to providing precise directory metadata, GROW offers two additional advantages over plain HTTP:

- Aggressive Caching. GROW caches files in an on-disk cache, but unlike plain HTTP, does not need to issue up-to-date checks against the server. Using the cached directory metadata, it can tell if a file is up-to-date without any network communication. The directory is only checked for changes at the beginning of program execution, so changes become visible only to newly executed programs.
- SHA-1 Integrity. `make_growfs` generates SHA-1 checksums on the directory and each file so that the integrity of the system can be verified at runtime. If a checksum fails, GROW will attempt to reload the file or directory listing in order to repair the error, trying until the master timeout (set by the -T option) expires. This will also occur if the underlying files have been modified and `make_growfs` has not yet been re-run. If necessary, checksums can be disabled by giving the `-k` option to either Parrot or `make_growfs`.

  ### iRODS⇗

  To use Parrot with iRODS, you must first follow the special build instructions.

  Then, use the `iinit` command to log into the desired iRODS service, and verify that you are connected with the `ils` command. If those work, then you can use Parrot to access files under the scheme `/irods/server/zone/path`. For example, to access the iPlant data service:

      parrot_run bash
      cd /irods/data.iplantcollaborative.org/iplant/home/username
      cp /tmp/mydata .

  ### CVMFS - CernVM-FS⇗

  CVMFS is a read-only filesystem, which was initially based on GROW. It is used within CernVM-FS to provide access to software repositories. It may be used outside of CernVM-FS by mounting it as a FUSE module. Parrot makes it possible to access CVMFS in cases where mounting CVMFS via FUSE is not an option. Like GROW, CVMFS makes use of web proxies and local disk caching for scalability. For security, data integrity is verified with cryptographic checksums. For increased reliability and performance, CVMFS repositories may also be mirrored in multiple locations and accessed via groups of load-balanced web proxies, with fail-over between groups.

  The CVMFS repositories hosted by the CernVM project and by Open Science Grid (OASIS) are enabled by default. To access a different repository, it is necessary to configure parrot to know how to access the repository. This may be done with the `-r` option or with the `PARROT_CVMFS_REPO` environment variable. The repository configuration syntax is `repo_name:options repo_name2:option2 ...`

  The repository with `repo_name` is used when the parrot user attempts to access the matching path `/cvmfs/repo_name/...`. The configured repository name may begin with `*`, which acts as a wildcard, matching one or more characters in the requested repository name. This is useful when multiple repositories are hosted at the same site and all configuration details are the same except the beginnings of the repository names, as in `atlas.cern.ch` and `cms.cern.ch`. Any `*` appearing in the options is replaced by the characters in the requested path that were matched by the `*` in the configured repository name. If a cvmfs path matches more than one configured repository, the last one appearing in the configuration takes precedence.

  The format of the repository options is `option1=value1,option2=value2,...`

  Literal spaces, tabs, newlines, asterisks, equal signs, or commas in the options must be proceeded by a backslash to avoid being interpreted as delimiters. If the same option is specified more than once, the last value takes precedence. The possible options are listed in the table below. The `url` option is required and has no default. The `proxies` option is required and defaults to the proxy used by parrot, if any.

  url=URL | The URL of the CernVM-FS server(s): 'url1;url2;...'
  proxies=HTTP_PROXIES | Set the HTTP proxy list, such as 'proxy1|proxy2'; default is given by -P option (HTTP_PROXY)
  | Proxies separated by '|' are randomly chosen for load balancing. Groups of proxies separated by ';' may be specified for failover. If the first group fails, the second group is used, and so on down the chain.
  cachedir=DIR | Where to store disk cache; default is within parrot temp directory (-t option)
  timeout=SECONDS | Timeout for network operations; default is given by -T option (PARROT_TIMEOUT)
  timeout_direct=SECONDS | Timeout in for network operations without proxy; default is given by -T option (PARROT_TIMEOUT)
  max_ttl=MINUTES | Maximum TTL for file catalogs; default: take from catalog
  allow_unsigned | Accept unsigned catalogs (allows man-in-the-middle attacks)
  whitelist=URL | HTTP location of trusted catalog certificates (defaults is /.cvmfswhitelist)
  pubkey=PEMFILE | Public RSA key that is used to verify the whitelist signature.
  rebuild_cachedb | Force rebuilding the quota cache db from cache directory
  quota_limit=MB | Limit size of cache. -1 (the default) means unlimited.
  | If not -1, files larger than quota_limit-quota_threshold will not be readable.
  quota_threshold=MB | Cleanup cache until size is <= threshold
  deep_mount=prefix | Path prefix if a repository is mounted on a nested catalog
  repo_name=NAME | Unique name of the mounted repository; default is the name used for this configuration entry
  mountpoint=PATH | Path to root of repository; default is /cvmfs/repo_name
  blacklist=FILE | Local blacklist for invalid certificates. Has precedence over the whitelist.
  try_local_filesystem | If this cvmfs repository is mounted on the local filesystem, use that instead of Parrot's CVMFS client.

  Setting the CVMFS configuration overrides the default configuration. If it is desired to configure additional repositories but still retain the default repositories, the configuration entry `<default-repositories>` may be put in the configuration string. Example: `parrot_run -r '<default-repositories> my.repo:url=http://cvmfs.server.edu/cvmfs/my.repo.edu' ...`

  `<default-repositories>` should come first, because it contains a catch-all clause `*:try_local_filesystem` that matches anything that isn't caught by later entries. This clause allows access to locally mounted CVMFS repositories that Parrot is not configured to access internally.

  The configuration of the default repositories may be modified by specifying additional options using the syntax `<default-repositories>:option1=value1,option2=value2`.

  The CVMFS library can only support one active repository open at a time. If a program attempts to access more than one repository in a session, Parrot will emit an error. If the optional argument --cvmfs-repo-switching is given to `parrot_run`, then Parrot will switch repositories at run-time by re-initializing the CVMFS library. This feature is experimental.

  ### Parrot Cache⇗

  If a service does not allow partial reads of a file (e.g. the HTTP protocol: `/http/foo.com/a.txt`), then Parrot will cache an entire copy of the file in its temporary directory (`-t` or `$PARROT_TEMP_DIR` or `$TMPDIR/parrot.<pid>`, in order). Cached files are named based on the hash of the canonical file name.

  You can also force Parrot to cache all non-local files using the `-F/--with-snapshots` option. This is not usually recommended because Parrot will download the entire copy of the file when opened by the application. This also means that updates to the file during run-time are ignored (hence the name, "snapshot")!

  Some services have their own cache, like `cvmfs`. This is a cache independent of the regular Parrot cache. It is important to note that some versions of CVMFS may not correctly operate on the same cache. In that case, it is essential to run concurrent instances of Parrot with different temporary directories.

  ### Hadoop Distributed File System (HDFS)⇗

  HDFS is the primary distributed filesystem used in the Hadoop project. Parrot supports read and write access to HDFS systems using the `parrot_run_hdfs` wrapper. This script checks that the appropriate environmental variables are defined and calls `parrot`.

  In particular, you must ensure that you define the following environmental variables: `JAVA_HOME Location of your Java installation. HADOOP_HOME Location of your Hadoop installation.`

  Based on these environmental variables, `parrot_run_hdfs` will attempt to find the appropriate paths for `libjvm.so` and `libhdfs.so`. These paths are stored in the environmental variables `LIBJVM_PATH` and `LIBHDFS_PATH`, which are used by the HDFS Parrot module to load the necessary shared libraries at run-time. To avoid the startup overhead of searching for these libraries, you may set the paths manually in your environment before calling `parrot_run_hdfs`, or you may edit the script directly.

  Note that while Parrot supports read access to HDFS, it only provides write-once support on HDFS. This is because the current implementations of HDFS do not provide reliable append operations. Likewise, files can only be opened in either read (`O_RDONLY`) or write mode (`O_WRONLY`), and not both (`O_RDWR`).

  ## Identity Boxing⇗

  Parot provides a unique feature known as identity boxing. This feature allows you to run a (possibly) untrusted program within a protection domain, as if it were run in a completely separate account. Using an identity box, you do not need to become root or even to manage account names: you can create any identity that you like on the fly.

  For example, suppose that you wish to allow a friend to log into your private workstation. Instead of creating a new account, simply use a script supplied with Parrot to create an identity box:

  `% whoami dthain % parrot_identity_box MyFriend % whoami MyFriend % touch ~dthain/private-data touch: creating ~dthain/private-data': Permission denied`

  Note that the shell running within the identity box cannot change or modify any of the supervising user's data. In fact, the contained user can only access items that are world-readable or world-writable.

  You can give the contained user access to other parts of the filesystem by creating access control lists. (ACLs) An ACL is a list of users and the resources that they are allowed to access. Each directory has it's own ACL in the file `.__acl`. This file does not appear in a directory listing, but you can read and write it just the same.

  For example, `MyFriend` above can see his initial ACL as follows:

  `% cat .__acl MyFriend rwlxa`

  This means that `MyFriend` can read, write, list, execute, and administer items in the current directory. Now, suppose that `MyFriend` wants to allow `Freddy` read access to the same directory. Simply edit the ACL file to read: `MyFriend rwlxa Freddy rl` Identity boxing and ACLs are particularly useful when using distributed storage. You can read more about ACLs and identity boxing in the Chirp manual.

  ## 64-Bit Support⇗

  In all modes, Parrot supports applications that access large (>2GB) files that require 64-bit seek pointers. However, we have found that many tools and filesystems do not manipulate such large files properly. If possible, we advise users to break up files into smaller pieces for processing.

  Parrot supports 64 bit programs and processors in the following combinations:

  Program Type | CPU Type
  --- | ---
  32-bit | 64-bit
  YES | NO | Parrot for 32-bit X86 CPU Pentium, Xeon, Athlon, Sempron
  YES | YES | Parrot for 64-bit X86_64 CPU Opteron, Athlon64, Turion64, Sempron64

  ## parrot inside docker⇗

  Docker by default blocks ptrace, the system call on which parrot relies. To run parrot inside docker, the container needs to be started using the `--security-opt seccomp=unconfined` command line argument. For example: `docker run --security-opt seccomp=unconfined MY-DOCKER-IMAGE`

  ## For More Information⇗

  For the latest information on Parrot, visit the home page:

- Parrot Home Page

  An exhaustive list of all options and commands can be found in the manual pages:

- parrot_run
- parrot_run_hdfs
- parrot_cp
- parrot_md5
- parrot_getacl
- parrot_setacl
- parrot_mkalloc
- parrot_lsalloc
- parrot_locate
- parrot_timeout
- parrot_whoami
- parrot_mount