What are the three operating modes of linux cpio

cpio has three operating modes: 1. Copy-out mode, used to package backup files and copy the files to the archive package; 2. Copy-in mode, used to restore files from archive package files , you can read the archive package from the standard input, read the files in the archive package or list the contents of the archive package; 3. The copy-pass mode is used to copy files from one directory tree to another.

#The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

In Linux, cpio is a data backup and recovery tool that is used to create cpio archive files, extract files from archive files, or copy files from one directory to another. The generated archive file can be a disk file, or it can be written to a data streaming medium such as tape. cpio can not only process cpio archive files in binary and ASCII header data formats, but can also read and write archive files in various tar formats. To maintain compatibility, cpio usually still creates archive files in binary header data format unless otherwise specified.

cpio is similar to the tar command. It can copy and package files into archive package files in cpio or tar format, or copy files from archive package files. Archive package files can be files on disk or device files such as tapes.

When extracting files from archive files, the cpio command can automatically identify which archive file is being read, and can also read archive files with different byte orders created in different systems.

cpio has three operating modes, as shown in the following table:

cpio three operating modes

cpio Three operating modes
Operation mode	Mode description
copy-out mode	This mode Used to package backup files.                            cpio copies the files into the archive package. The file names to be packed by cpio come from standard input, and it gets a list of file names from standard input, one per line. Generally, the standard input to the output of the standard input to the other command is generally through the pipeline. The most common method is to use the find command to generate a list of file names and then pipe it to cpio. This way cpio can know where the data to be backed up comes from. You need to add the -depth option after find to reduce the trouble caused by entering directories without access permissions. By default, cpio writes archive package files to standard output, so you need to use the redirection symbol > or >> to redirect the archive package data output to a file.
copy-in mode	This mode is used to restore files from archived package files. cpio reads the archive package from standard input, reads the files in the archive package, or lists the contents of the archive package. cpio writes the data read from the archive package to standard output.
copy-pass mode	cpio copies files from one directory tree to another. It combines the copy-in and copy-out operations, but Archive packages are not used. CPIO is given a list of files for reading from standard input; the target directory is given as a command line parameter for non -options.

cpio supports the following archive formats: binary, old ASCII, new ASCII, crc, HPUX binary, HPUX old ASCII, old tar, and POSIX.1 tar.

Some options of cpio can only be used in the corresponding operating mode. The commonly used command option format is as follows:

[root@initroot ~]# cpio -ovcB > [file|device] #备份
[root@initroot ~]# cpio -ivcdu < [file|device] #还原
[root@initroot ~]# cpio -ivct < [file|device] #察看

copy-out mode (backup) Options and parameters used:

• #-o: Copy the data to a file or device

• -B: Let The default Blocks can be increased to 5120bytes, and the default is 512bytes! The advantage of this is that it can speed up the storage of large files (please refer to the concept of i-nodes)

Options and parameters used in copy-in mode (restore):

• #-i: Copy data from a file or device out of the system

• -d: Automatically create a directory! The data backed up using cpio may not be in the same directory, so we must allow cpio to create a new directory during restore. In this case, the -d option is required Help!

• -u: Automatically overwrite older files with newer files!

• #-t: Requires -i option , can be used to "view" the contents of files or devices created with cpio

Common options and parameters:

• -v: Allow the file name to be displayed on the screen during the saving process

• -c: A newer portable format storage method

cpio reads data from the standard input and outputs the processed data to the standard output, so cpio needs to be used with pipes and redirection symbols.

Find all the files in the /boot directory and back them up to the /tmp/boot.cpio archive file:

[root@initroot ~]# cd /
[root@initroot /]# find boot -print
boot
boot/grub
boot/grub/gfxblacklist.txt
boot/grub/unicode.pf2
boot/grub/locale
...省略...

The find command can find all the files in the boot directory, including files With the directory! Note that the boot here is a relative path, not an absolute path!

[root@initroot /]# find boot | cpio -ocvB > /tmp/boot.cpio

[root@initroot /]# ls -lh /tmp/boot.cpio
-rw-r--r-- 1 peter peter 193M Feb 10 15:59 /tmp/boot.cpio
[root@initroot ~]# file /tmp/boot.cpio
/tmp/boot.cpio: ASCII cpio archive (pre-SVR4 or odc)

Use find boot to find the file name, pass the output to cpio through the pipeline |, and archive the data through the redirection symbol > Output to the /tmp/boot.cpio file! The parameter boot found above is a relative path, not an absolute path. cpio will not do special processing for the path. It will receive whatever data is given. So if an absolute path is used here, then the output of the find command is also an absolute path, and what cpio receives is also an absolute path. Then when the file is copied from the archive file, because it is an absolute path, the copied file will definitely be overwritten. Delete the files in the original /boot directory! This is very dangerous! Therefore, when using the find command with cpio to package files, be sure to use relative paths. Refer to the -P option of the tar command!!

Now unzip the archive package file /tmp/boot.cpio to the /root/ directory:

[root@initroot ~]# cd ~
[root@initroot ~]# cpio -idvc < /tmp/boot.cpio
[root@initroot ~]# ls -al /root/boot

You can see that /root/boot and The contents of /boot are exactly the same!

cpio can completely back up the system data to the tape drive:

[root@initroot ~]# find / | cpio -ocvB > /dev/st0

Restore data from the tape drive:

[root@initroot ~]# cpio -idvc < /dev/st0

cpio can back up any Files, including device files under /dev! It is a very important command!

cpio must cooperate with other programs, such as find to create file names, so cpio has very important differences with pipeline commands and data flow redirection. Close relationship!

In fact, the /boot/initramfs-xxx or /boot/initrd.img-xxx-generic file in the /boot directory is also a cpio archive file.

We can unzip the file to the /tmp/initramfs directory to see what files are in this archive package:

[root@initroot ~]# file initrd.img-4.15.0-65-generic
initrd.img-4.15.0-65-generic: ASCII cpio archive (SVR4 with no CRC)
[root@initroot ~]# mkdir /tmp/initramfs
[root@initroot ~]# cd /tmp/initramfs
[root@initroot initramfs]# cpio -idv < /boot/initrd.img-4.15.0-65-generic
.
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/AuthenticAMD.bin
56 blocks

Note that the -c option is removed here. If you add - If the c option is used, cpio will display the following prompt:

cpio: premature end of file

Remove the -c option to successfully decompress the archive package file!

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