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DEBUGFS(8) – System Manager's Manual – DEBUGFS(8)

📖 NAME

debugfs – ext2/ext3/ext4 file system debugger

🚀 Quick Reference

Use Case	Command	Description
🔍 Examine on‑disk structures	debugfs /dev/sdXX	Open a file system in read‑only mode for inspection
✏️ Modify a file system	debugfs –w /dev/sdXX	Open a file system in read‑write mode
📜 Run a script of commands	debugfs –f cmd_file /dev/sdXX	Execute commands from a file, then exit
⚡ Execute a single command	debugfs –R "request" /dev/sdXX	Run one debugfs command and exit
📂 List directory contents	ls	Print files in the current directory
📄 Dump file contents	cat filespec	Write the contents of an inode to stdout
🗑️ Recover deleted files	undel <inode_number> [pathname]	Undelete an inode and optionally link it
🔧 Modify inode fields	set_inode_field filespec field value	Change an inode attribute (e.g., i_size)

🔍 SYNOPSIS

debugfs [ -DVwcin ] [ -b blocksize ] [ -s superblock ] [ -f cmd_file ] [ -R request ] [ -d data_source_device ] [ -z undo_file ] [ device ]

📝 DESCRIPTION

The debugfs program is an interactive file system debugger. It can be used to examine and change the state of an ext2, ext3, or ext4 file system.

device is a block device (e.g., /dev/sdXX) or a file containing the file system.

⚙️ OPTIONS

• -w ✏️ Specifies that the file system should be opened in read‑write mode. Without this option, the file system is opened in read‑only mode.
• -n 🚫 Disables metadata checksum verification. This should only be used if you believe the metadata to be correct despite the complaints of e2fsprogs.
• -c 💥 Specifies that the file system should be opened in catastrophic mode, in which the inode and group bitmaps are not read initially. This can be useful for file systems with significant corruption, but because of this, catastrophic mode forces the file system to be opened read‑only.
• -i 🖼️ Specifies that device represents an ext2 image file created by the e2image program. Since the ext2 image file only contains the superblock, block group descriptor, block and inode allocation bitmaps, and the inode table, many debugfs commands will not function properly. Warning: no safety checks are in place, and debugfs may fail in interesting ways if commands such as ls, dump, etc. are tried without specifying the data_source_device using the -d option. debugfs is a debugging tool. It has rough edges!
• -d data_source_device 📁 Used with the -i option, specifies that data_source_device should be used when reading blocks not found in the ext2 image file. This includes data, directory, and indirect blocks.
• -b blocksize 📏 Forces the use of the given block size (in bytes) for the file system, rather than detecting the correct block size automatically. (This option is rarely needed; it is used primarily when the file system is extremely badly damaged/corrupted.)
• -s superblock 🧩 Causes the file system superblock to be read from the given block number, instead of using the primary superblock (located at an offset of 1024 bytes from the beginning of the file system). If you specify the -s option, you must also provide the blocksize of the file system via the -b option. (This option is rarely needed; it is used primarily when the file system is extremely badly damaged/corrupted.)
• -f cmd_file 📜 Causes debugfs to read in commands from cmd_file, and execute them. When debugfs is finished executing those commands, it will exit.
• -D ⚡ Causes debugfs to open the device using Direct I/O, bypassing the buffer cache. Note that some Linux devices, notably device mapper as of this writing, do not support Direct I/O.
• -R request 🎯 Causes debugfs to execute the single command request, and then exit.
• -V ℹ️ print the version number of debugfs and exit.
• -z undo_file ⏪ Before overwriting a file system block, write the old contents of the block to an undo file. This undo file can be used with e2undo(8) to restore the old contents of the file system should something go wrong. If the empty string is passed as the undo_file argument, the undo file will be written to a file named debugfs-device.e2undo in the directory specified via the E2FSPROGS_UNDO_DIR environment variable. WARNING: The undo file cannot be used to recover from a power or system crash.

📂 SPECIFYING FILES

Many debugfs commands take a filespec as an argument to specify an inode (as opposed to a pathname) in the file system which is currently opened by debugfs. The filespec argument may be specified in two forms. The first form is an inode number surrounded by angle brackets, e.g., <2>. The second form is a pathname; if the pathname is prefixed by a forward slash ('/'), then it is interpreted relative to the root of the file system which is currently opened by debugfs. If not, the pathname is interpreted relative to the current working directory as maintained by debugfs. This may be modified by using the debugfs command cd.

🛠️ COMMANDS

This is a list of the commands which debugfs supports.

• blocks filespec – Print the blocks used by the inode filespec to stdout.
• bmap [ -a ] filespec logical_block [physical_block] – Print or set the physical block number corresponding to the logical block number logical_block in the inode filespec. If the -a flag is specified, try to allocate a block if necessary.
• block_dump [-x] [-f filespec] block_num – Dump the file system block given by block_num in hex and ASCII format to the console. If the -f option is specified, the block number is relative to the start of the given filespec. If the -x option is specified, the block is interpreted as an extended attribute block and printed to show the structure of extended attribute data structures.
• cat filespec – Dump the contents of the inode filespec to stdout.
• cd filespec – Change the current working directory to filespec.
• chroot filespec – Change the root directory to be the directory filespec.
• close [-a] – Close the currently open file system. If the -a option is specified, write out any changes to the superblock and block group descriptors to all of the backup superblocks, not just to the master superblock.
• clri filespec – Clear the contents of the inode filespec.
• copy_inode source_inode destination_inode – Copy the contents of the inode structure in source_inode and use it to overwrite the inode structure at destination_inode.
• dirsearch filespec filename – Search the directory filespec for filename.
• dirty [-clean] – Mark the file system as dirty, so that the superblocks will be written on exit. Additionally, clear the superblock's valid flag, or set it if -clean is specified.
• dump [-p] filespec out_file – Dump the contents of the inode filespec to the output file out_file. If the -p option is given set the owner, group and permissions information on out_file to match filespec.
• dump_mmp [mmp_block] – Display the multiple‑mount protection (mmp) field values. If mmp_block is specified then verify and dump the MMP values from the given block number, otherwise use the s_mmp_block field in the superblock to locate and use the existing MMP block.
• dx_hash [-h hash_alg] [-s hash_seed] filename – Calculate the directory hash of filename. The hash algorithm specified with -h may be legacy, half_md4, or tea. The hash seed specified with -s must be in UUID format.
• dump_extents [-n] [-l] filespec – Dump the extent tree of the inode filespec. The -n flag will cause dump_extents to only display the interior nodes in the extent tree. The -l flag will cause dump_extents to only display the leaf nodes in the extent tree. (Please note that the length and range of blocks for the last extent in an interior node is an estimate by the extents library functions, and is not stored in file system data structures. Hence, the values displayed may not necessarily be accurate and does not indicate a problem or corruption in the file system.)
• dump_unused – Dump unused blocks which contain non‑null bytes.
• ea_get [-f outfile]|[-xVC] [-r] filespec attr_name – Retrieve the value of the extended attribute attr_name in the file filespec and write it either to stdout or to outfile.
• ea_list filespec – List the extended attributes associated with the file filespec to standard output.
• ea_set [-f infile] [-r] filespec attr_name attr_value – Set the value of the extended attribute attr_name in the file filespec to the string value attr_value or read it from infile.
• ea_rm filespec attr_names... – Remove the extended attribute attr_name from the file filespec.
• expand_dir filespec – Expand the directory filespec.
• fallocate filespec start_block [end_block] – Allocate and map uninitialized blocks into filespec between logical block start_block and end_block, inclusive. If end_block is not supplied, this function maps until it runs out of free disk blocks or the maximum file size is reached. Existing mappings are left alone.
• feature [fs_feature] [-fs_feature] ... – Set or clear various file system features in the superblock. After setting or clearing any file system features that were requested, print the current state of the file system feature set.
• filefrag [-dvr] filespec – Print the number of contiguous extents in filespec. If filespec is a directory and the -d option is not specified, filefrag will print the number of contiguous extents for each file in the directory. The -v option will cause filefrag print a tabular listing of the contiguous extents in the file. The -r option will cause filefrag to do a recursive listing of the directory.
• find_free_block [count [goal]] – Find the first count free blocks, starting from goal and allocate it. Also available as ffb.
• find_free_inode [dir [mode]] – Find a free inode and allocate it. If present, dir specifies the inode number of the directory which the inode is to be located. The second optional argument mode specifies the permissions of the new inode. (If the directory bit is set on the mode, the allocation routine will function differently.) Also available as ffi.
• freeb block [count] – Mark the block number block as not allocated. If the optional argument count is present, then count blocks starting at block number block will be marked as not allocated.
• freefrag [-c chunk_kb] – Report free space fragmentation on the currently open file system. If the -c option is specified then the filefrag command will print how many free chunks of size chunk_kb can be found in the file system. The chunk size must be a power of two and be larger than the file system block size.
• freei filespec [num] – Free the inode specified by filespec. If num is specified, also clear num‑1 inodes after the specified inode.
• get_quota quota_type id – Display quota information for given quota type (user, group, or project) and ID.
• help – Print a list of commands understood by debugfs.
• htree_dump filespec – Dump the hash‑indexed directory filespec, showing its tree structure.
• icheck block ... – Print a listing of the inodes which use the one or more blocks specified on the command line.
• inode_dump [-b]|[-e]|[-x] filespec – Print the contents of the inode data structure in hex and ASCII format. The -b option causes the command to only dump the contents of the i_blocks array. The -e option causes the command to only dump the contents of the extra inode space, which is used to store in‑line extended attributes. The -x option causes the command to dump the extra inode space interpreted and extended attributes. This is useful to debug corrupted inodes containing extended attributes.
• imap filespec – Print the location of the inode data structure (in the inode table) of the inode filespec.
• init_filesys device blocksize – Create an ext2 file system on device with device size blocksize. Note that this does not fully initialize all of the data structures; to do this, use the mke2fs(8) program. This is just a call to the low‑level library, which sets up the superblock and block descriptors.
• journal_close – Close the open journal.
• journal_open [-c] [-v ver] [-f ext_jnl] – Opens the journal for reading and writing. Journal checksumming can be enabled by supplying -c; checksum formats 2 and 3 can be selected with the -v option. An external journal can be loaded from ext_jnl.
• journal_run – Replay all transactions in the open journal.
• journal_write [-b blocks] [-r revoke] [-c] file – Write a transaction to the open journal. The list of blocks to write should be supplied as a comma‑separated list in blocks; the blocks themselves should be readable from file. A list of blocks to revoke can be supplied as a comma‑separated list in revoke. By default, a commit record is written at the end; the -c switch writes an uncommitted transaction.
• kill_file filespec – Deallocate the inode filespec and its blocks. Note that this does not remove any directory entries (if any) to this inode. See the rm(1) command if you wish to unlink a file.
• lcd directory – Change the current working directory of the debugfs process to directory on the native file system.
• list_quota quota_type – Display quota information for given quota type (user, group, or project).
• ln filespec dest_file – Create a link named dest_file which is a hard link to filespec. Note this does not adjust the inode reference counts.
• logdump [-acsOS] [-b block] [-i filespec] [-f journal_file] [output_file] – Dump the contents of the ext3 journal. By default, dump the journal inode as specified in the superblock. However, this can be overridden with the -i option, which dumps the journal from the internal inode given by filespec. A regular file containing journal data can be specified using the -f option. Finally, the -s option utilizes the backup information in the superblock to locate the journal. The -S option causes logdump to print the contents of the journal superblock. The -a option causes the logdump program to print the contents of all of the descriptor blocks. The -b option causes logdump to print all journal records that refer to the specified block. The -c option will print out the contents of all of the data blocks selected by the -a and -b options. The -O option causes logdump to display old (checkpointed) journal entries. This can be used to try to track down journal problems even after the journal has been replayed.
• ls [-l] [-c] [-d] [-p] [-r] filespec – Print a listing of the files in the directory filespec. The -c flag causes directory block checksums (if present) to be displayed. The -d flag will list deleted entries in the directory. The -l flag will list files using a more verbose format. The -p flag will list the files in a format which is more easily parsable by scripts, as well as making it more clear when there are spaces or other non‑printing characters at the end of filenames. The -r flag will force the printing of the filename, even if it is encrypted.
• list_deleted_inodes [limit] – List deleted inodes, optionally limited to those deleted within limit seconds ago. Also available as lsdel. This command was useful for recovering from accidental file deletions for ext2 file systems. Unfortunately, it is not useful for this purpose if the files were deleted using ext3 or ext4, since the inode's data blocks are no longer available after the inode is released.
• modify_inode filespec – Modify the contents of the inode structure in the inode filespec. Also available as mi.
• mkdir filespec – Make a directory.
• mknod filespec [p|[[c|b] major minor]] – Create a special device file (a named pipe, character or block device). If a character or block device is to be made, the major and minor device numbers must be specified.
• ncheck [-c] inode_num ... – Take the requested list of inode numbers, and print a listing of pathnames to those inodes. The -c flag will enable checking the file type information in the directory entry to make sure it matches the inode's type.
• open [-weficD] [-b blocksize] [-d image_filename] [-s superblock] [-z undo_file] device – Open a file system for editing. The -f flag forces the file system to be opened even if there are some unknown or incompatible file system features which would normally prevent the file system from being opened. The -e flag causes the file system to be opened in exclusive mode. The -b, -c, -d, -i, -s, -w, and -D options behave the same as the command‑line options to debugfs.
• punch filespec start_blk [end_blk] – Delete the blocks in the inode ranging from start_blk to end_blk. If end_blk is omitted then this command will function as a truncate command; that is, all of the blocks starting at start_blk through to the end of the file will be deallocated.
• symlink filespec target – Make a symbolic link.
• pwd – Print the current working directory.
• quit – Quit debugfs.
• rdump directory[...] destination – Recursively dump directory, or multiple directories, and all its contents (including regular files, symbolic links, and other directories) into the named destination, which should be an existing directory on the native file system.
• rm pathname – Unlink pathname. If this causes the inode pointed to by pathname to have no other references, deallocate the file. This command functions as the unlink() system call.
• rmdir filespec – Remove the directory filespec.
• setb block [count] – Mark the block number block as allocated. If the optional argument count is present, then count blocks starting at block number block will be marked as allocated.
• set_block_group bgnum field value – Modify the block group descriptor specified by bgnum so that the block group descriptor field field has value value. Also available as set_bg.
• set_current_time time – Set current time in seconds since Unix epoch to use when setting file system fields.
• seti filespec [num] – Mark inode filespec as in use in the inode bitmap. If num is specified, also set num‑1 inodes after the specified inode.
• set_inode_field filespec field value – Modify the inode specified by filespec so that the inode field field has value value. The list of valid inode fields which can be set via this command can be displayed by using the command: set_inode_field -l. Also available as sif.
• set_mmp_value field value – Modify the multiple‑mount protection (MMP) data so that the MMP field field has value value. The list of valid MMP fields which can be set via this command can be displayed by using the command: set_mmp_value -l. Also available as smmp.
• set_super_value field value – Set the superblock field field to value. The list of valid superblock fields which can be set via this command can be displayed by using the command: set_super_value -l. Also available as ssv.
• show_debugfs_params – Display debugfs parameters such as information about currently opened file system.
• show_super_stats [-h] – List the contents of the super block and the block group descriptors. If the -h flag is given, only print out the superblock contents. Also available as stats.
• stat filespec – Display the contents of the inode structure of the inode filespec.
• supported_features – Display file system features supported by this version of debugfs.
• testb block [count] – Test if the block number block is marked as allocated in the block bitmap. If the optional argument count is present, then count blocks starting at block number block will be tested.
• testi filespec – Test if the inode filespec is marked as allocated in the inode bitmap.
• undel <inode_number> [pathname] – Undelete the specified inode number (which must be surrounded by angle brackets) so that it and its blocks are marked in use, and optionally link the recovered inode to the specified pathname. The e2fsck command should always be run after using the undel command to recover deleted files. Note that if you are recovering a large number of deleted files, linking the inode to a directory may require the directory to be expanded, which could allocate a block that had been used by one of the yet‑to‑be‑undeleted files. So it is safer to undelete all of the inodes without specifying a destination pathname, and then in a separate pass, use the debugfs link command to link the inode to the destination pathname, or use e2fsck to check the file system and link all of the recovered inodes to the lost+found directory.
• unlink pathname – Remove the link specified by pathname to an inode. Note this does not adjust the inode reference counts.
• write source_file out_file – Copy the contents of source_file into a newly‑created file in the file system named out_file.
• zap_block [-f filespec] [-o offset] [-l length] [-p pattern] block_num – Overwrite the block specified by block_num with zero (NUL) bytes, or if -p is given use the byte specified by pattern. If -f is given then block_num is relative to the start of the file given by filespec. The -o and -l options limit the range of bytes to zap to the specified offset and length relative to the start of the block.
• zap_block [-f filespec] [-b bit] block_num – Bit‑flip portions of the physical block_num. If -f is given, then block_num is a logical block relative to the start of filespec.

🌐 ENVIRONMENT VARIABLES

• DEBUGFS_PAGER, PAGER – The debugfs program always pipes the output of some commands through a pager program. These commands include: show_super_stats (stats), list_directory (ls), show_inode_info (stat), list_deleted_inodes (lsdel), and htree_dump. The specific pager can be explicitly specified by the DEBUGFS_PAGER environment variable, and if it is not set, by the PAGER environment variable. Note that since a pager is always used, the less(1) pager is not particularly appropriate, since it clears the screen before displaying the output of the command and clears the output the screen when the pager is exited. Many users prefer to use the less(1) pager for most purposes, which is why the DEBUGFS_PAGER environment variable is available to override the more general PAGER environment variable.

👤 AUTHOR

debugfs was written by Theodore Ts'o <tytso@mit.edu>.

📚 SEE ALSO

dumpe2fs(8), tune2fs(8), e2fsck(8), mke2fs(8), ext4(5)

E2fsprogs version 1.46.5 – December 2021 – DEBUGFS(8)

📖 NAME 🚀 Quick Reference 🔍 SYNOPSIS 📝 DESCRIPTION ⚙️ OPTIONS 📂 SPECIFYING FILES 🛠️ COMMANDS 🌐 ENVIRONMENT VARIABLES 👤 AUTHOR 📚 SEE ALSO