file 中文man页面

名称

file - 确定文件类型  

总览

file [ -bcnsvzL ] [ -f 命名文件 ] [ -m 幻数文件 ] file ...  

描述

本手册页说明了3.27版本 file 命令的使用. File 命令试图检查每个参数以判定文件的类型. 检查共有三组，按如下顺序进行:文件系统检查，幻数检查，以及语言检查. 文件系统 检查成功则输出文件类型.

输出的类型一般会包含以下的词中的一个： text (文件中仅有 ASCII 字符，可以用 ASCII 终端读此文件，以保证内容的可靠性), executable (文件中保存的是程序编译后的结果，一些UNIX 内核或其它内核能理解这类文件), 或者 data 表示所有其它类型文件(data 一般为二进制文件或者不可打印的). 但是有的常用的文件格式（如core文件、tar包），虽然也包含二进制数据，却不属于这一类如果要修改 /usr/share/magic 或者程序本身, preserve these keywords . 当文件为``text'' 类型时，认为此文件为可读文件. 不要象在Berkeley环境中那样做 - 要把``shell commands text''改为``shell script''.

文件系统检查是建立在对 stat(2) 系统调用结果的分析上的. 程序会分析文件是否为空,或者是否是某种特殊文件. 对于所有可在现有系统上使用的文件类型 (比如套接口文件，动态链接文件，命名管道文件(FIFOs) 等)，只要它在系统头文件 sys/stat.h 中已经定义过，就可以被检查到.

幻数检查用来检查文件中是否有特殊的固定格式的数据. 规范的例子如二进制可执行文件(编译后的程序) a.out ,该文件格式在标准include目录下的 a.out.h 文件中定义，也可能在 exec.h 中定义. 这些文件在文件开始部分附近的一个特殊位置保存有一个'幻数' , 通过幻数告诉UNIX 操作系统此文件是二进制可执行文件, 和其中包含的其它类型. 幻数的概念已经扩展到数据文件.任何在文件固定位置有与文件类型相关的不变标识符的文件都可以这样表示. 这些文件中的信息可以从幻数文件 /usr/share/magic 中读取.

如果文件为 ASCII 文件, file 会试图检查它的语言. 语言检查在文件开始的几个块中(任意位置)查找是否有特殊字符串(参看 names.h) .br 指出此文件很可能是 troff(1) 输入文件, 而关键字 struct 指出此文件是C程序. 语言检查不如前两组检查可靠，所以放在最后执行.它也用来检查一些混合文件(例如 tar(1) 存档文件)并确定文件是`ascii text'类型还是`data'类型.  

选项

-b

不输出文件名 (简要模式).

-c

检查时打印输出幻数文件的解析结果.常与 -m 一起使用，用来在安装幻数文件之前调试它.

-f 命名文件

从在参数表前的 命名文件 中读出将要检查的文件名(每行一个文件).要有 命名文件 ，或者至少有一个文件名参数; 如果要检查标准输入, 使用``-''作为文件参数.

-m list

指定包含幻数的文件列表.可以是单个文件，也可以是用冒号分开的多个文件.

-n

每检查完一个文件就强制刷新标准输出. 仅在检查一组文件时才有效. 一般在将文件类型输出到管道时才采用此选项.

-v

打印程序版本并退出.

-z

试图查看压缩文件内部信息.

-L

(在支持符号链接的系统上)选项显示符号链接文件的原文件, 就像 ls(1) 命令的like-named 选项.

-s

通常, file 只是试图去检查在文件列表中那些 stat(2) 报告为正常文件的文件的类型.由于读特殊文件将可能导致不可知后果，所以这样可以防止发生问题.使用 -s 选项时 file 命令也将去读文件列表中的块特殊文件和字符特殊文件. 一般用于从原始磁盘分区中获得文件系统类型，此文件为块特殊文件. 这个选项也导致 file 命令忽略 stat(2) 报告的文件大小，因为在有些系统中原始磁盘分区的大小报告为0.

文件

/usr/share/magic - 默认的幻数列表  

ENVIRONMENT

环境变量 MAGIC 用于设置默认的幻数文件.  

参看

magic(4) - 幻数文件的格式.
strings(1), od(1), hexdump(1) - 检查非textfile的工具.  

#p#

NAME

file - determine file type  

SYNOPSIS

file [ -bcikLnNprsvz ] [ -f namefile ] [ -F separator ] [ -m magicfiles ] file ...
file -C [ -m magicfile ]  

DESCRIPTION

This manual page documents version 4.10 of the file command.

File tests each argument in an attempt to classify it. There are three sets of tests, performed in this order: filesystem tests, magic number tests, and language tests. The first test that succeeds causes the file type to be printed.

The type printed will usually contain one of the words text (the file contains only printing characters and a few common control characters and is probably safe to read on an ASCII terminal), executable (the file contains the result of compiling a program in a form understandable to some UNIX kernel or another), or data meaning anything else (data is usually `binary' or non-printable). Exceptions are well-known file formats (core files, tar archives) that are known to contain binary data. When modifying the file /usr/share/file/magic or the program itself, preserve these keywords . People depend on knowing that all the readable files in a directory have the word ``text'' printed. Don't do as Berkeley did and change ``shell commands text'' to ``shell script''. Note that the file /usr/share/file/magic is built mechanically from a large number of small files in the subdirectory Magdir in the source distribution of this program.

The filesystem tests are based on examining the return from a stat(2) system call. The program checks to see if the file is empty, or if it's some sort of special file. Any known file types appropriate to the system you are running on (sockets, symbolic links, or named pipes (FIFOs) on those systems that implement them) are intuited if they are defined in the system header file <sys/stat.h>.

The magic number tests are used to check for files with data in particular fixed formats. The canonical example of this is a binary executable (compiled program) a.out file, whose format is defined in a.out.h and possibly exec.h in the standard include directory. These files have a `magic number' stored in a particular place near the beginning of the file that tells the UNIX operating system that the file is a binary executable, and which of several types thereof. The concept of `magic number' has been applied by extension to data files. Any file with some invariant identifier at a small fixed offset into the file can usually be described in this way. The information identifying these files is read from the compiled magic file /usr/share/file/magic.mgc , or /usr/share/file/magic if the compile file does not exist.

If a file does not match any of the entries in the magic file, it is examined to see if it seems to be a text file. ASCII, ISO-8859-x, non-ISO 8-bit extended-ASCII character sets (such as those used on Macintosh and IBM PC systems), UTF-8-encoded Unicode, UTF-16-encoded Unicode, and EBCDIC character sets can be distinguished by the different ranges and sequences of bytes that constitute printable text in each set. If a file passes any of these tests, its character set is reported. ASCII, ISO-8859-x, UTF-8, and extended-ASCII files are identified as ``text'' because they will be mostly readable on nearly any terminal; UTF-16 and EBCDIC are only ``character data'' because, while they contain text, it is text that will require translation before it can be read. In addition, file will attempt to determine other characteristics of text-type files. If the lines of a file are terminated by CR, CRLF, or NEL, instead of the Unix-standard LF, this will be reported. Files that contain embedded escape sequences or overstriking will also be identified.

Once file has determined the character set used in a text-type file, it will attempt to determine in what language the file is written. The language tests look for particular strings (cf names.h) that can appear anywhere in the first few blocks of a file. For example, the keyword .br indicates that the file is most likely a troff(1) input file, just as the keyword struct indicates a C program. These tests are less reliable than the previous two groups, so they are performed last. The language test routines also test for some miscellany (such as tar(1) archives).

Any file that cannot be identified as having been written in any of the character sets listed above is simply said to be ``data''.  

OPTIONS

-b, --brief

Do not prepend filenames to output lines (brief mode).

-c, --checking-printout

Cause a checking printout of the parsed form of the magic file. This is usually used in conjunction with -m to debug a new magic file before installing it.

-C, --compile

Write a magic.mgc output file that contains a pre-parsed version of file.

-f, --files-from namefile

Read the names of the files to be examined from namefile (one per line) before the argument list. Either namefile or at least one filename argument must be present; to test the standard input, use ``-'' as a filename argument.

-F, --separator separator

Use the specified string as the separator between the filename and the file result returned. Defaults to ``:''.

-i, --mime

Causes the file command to output mime type strings rather than the more traditional human readable ones. Thus it may say ``text/plain; charset=us-ascii'' rather than ``ASCII text''. In order for this option to work, file changes the way it handles files recognised by the command itself (such as many of the text file types, directories etc), and makes use of an alternative ``magic'' file. (See ``FILES'' section, below).

-k, --keep-going

Don't stop at the first match, keep going.

-L, --dereference

option causes symlinks to be followed, as the like-named option in ls(1). (on systems that support symbolic links).

-m, --magic-file list

Specify an alternate list of files containing magic numbers. This can be a single file, or a colon-separated list of files. If a compiled magic file is found alongside, it will be used instead. With the -i or --mime option, the program adds ".mime" to each file name.

-n, --no-buffer

Force stdout to be flushed after checking each file. This is only useful if checking a list of files. It is intended to be used by programs that want filetype output from a pipe.

-N, --no-pad

Don't pad filenames so that they align in the output.

-p, --preserve-date

On systems that support utime(2) or utimes(2), attempt to preserve the access time of files analyzed, to pretend that file(2) never read them.

-r, --raw

Don't translate unprintable characters to \ooo. Normally file translates unprintable characters to their octal representation.

-s, --special-files

Normally, file only attempts to read and determine the type of argument files which stat(2) reports are ordinary files. This prevents problems, because reading special files may have peculiar consequences. Specifying the -s option causes file to also read argument files which are block or character special files. This is useful for determining the filesystem types of the data in raw disk partitions, which are block special files. This option also causes file to disregard the file size as reported by stat(2) since on some systems it reports a zero size for raw disk partitions.

-v, --version

Print the version of the program and exit.

-z, --uncompress

Try to look inside compressed files.

--help

Print a help message and exit.

FILES

/usr/share/file/magic.mgc

Default compiled list of magic numbers

/usr/share/file/magic

Default list of magic numbers

/usr/share/file/magic.mime.mgc

Default compiled list of magic numbers, used to output mime types when the -i option is specified.

/usr/share/file/magic.mime

Default list of magic numbers, used to output mime types when the -i option is specified.

/etc/magic

Local additions to magic wisdom.  

ENVIRONMENT

The environment variable MAGIC can be used to set the default magic number file name. file adds ".mime" and/or ".mgc" to the value of this variable as appropriate.  

SEE ALSO

magic(5) - description of magic file format.
strings(1), od(1), hexdump(1) - tools for examining non-textfiles.