Bash Index : M - Httqm's Docs

man is the utility displaying on-line reference manuals

Flag	Usage
-P pager	use pager to display pages and scroll up / down / page forward-backward / search / ...

Hack allowing to refer to any location within a manpage (source) :

man -P 'less -p ^READLINE' bash
man -P 'less -p ^EDITING' readline
LANG=C man -P 'less -p "^PROCESS STATE CODES"' ps

Add and remove modules from the Linux kernel

Flag	Usage
-a pattern	load all modules matching the pattern conditions
-c	display current configuration not equivalent to lsmod
-r moduleName	remove (=unload) the module moduleName
-t type	affect modules of the corresponding type Example, to list all i2c modules : modprobe -l -t i2c

MD5 (Message-Digest algorithm 5) was created for encryption purposes, but now, it's mostly used to check file integrity.
To compute the MD5 hash of myFile :

md5sum myFile

Check file integrity :

When downloading a large file (file.iso), you're also offered to retrieve a short plain text file usually named MD5SUM. This file contains the calculated md5 hashes of the file.iso in order to let you check its integrity after the download. To do so :

download both MD5SUM and file.iso into the same directory
check file integrity : md5sum -c MD5SUM
if the downloaded file.iso is an exact copy of the remote file.iso, it will display : file.iso: OK

Get the MD5 hash of an arbitrary string from the command line :

Single line string :

echo -n hello | md5sum

will return :

5d41402abc4b2a76b9719d911017c592 -

-n is required because echo appends a newline character to the output. Without this flag, the encoded string wouldn't be hello but hello\n, resulting in a different hash (source).

Multiple line string :

md5sum -
blablabla
bliblibli
blobloblo
CTRL-d
Should return : c3567beab1ff181e1d7b07d189437361 -

Make a directory

Flag	Usage
-m octalMask --mode=octalMask	Set file mode (as in chmod) using the mask octal notation format (i.e. : 644). Do not use the mask symbolic notation format (i.e. : u=rw,g=r,o=r), this may have undesired effects. There is no incompatibility between -m and -p, BUT if the target directory already exists : its mode will not be updated this will happen silently
-p --parents	Make parent directories as needed, return no error if existing If mkdir -p is intended to be followed by chown + chmod, have a look at install.

move and rename files

Flag	Usage	Details
-i --interactive	prompt before overwrite
-f --force	do not prompt before overwriting unclear which is the default: see -i and -n	echo LEAD > lead; echo GOLD > gold; mv lead gold; for i in lead gold; do echo -n "$i: "; cat "$i" && rm "$i"; done lead: cat: lead: No such file or directorymissing because renamed gold: LEAD gold has been overwritten without prompting
-n --no-clobber	do not overwrite an existing file When this applies, no UNIX_FAILURE is generated	echo LEAD > lead; echo GOLD > gold; mv -n lead gold; echo $?; cat lead gold; rm lead gold 0 UNIX_SUCCESS LEAD no file was altered GOLD

mv errors :

cannot move thisDirectory : Device or resource busy :

With lsof and fuser, make sure no process is actually accessing thisDirectory :
- It's likely there is an open shell which current directory is thisDirectory (in such situation, you'll find a Bash process using thisDirectory). Just cd somewhere else or terminate this process.
- Kill blocking processes (fuser -k ) then retry.
If no process is found but the error is still there, make sure the target directory is not a mount point.

mv: failed to preserve ownership for 'someFile': Permission denied (source) :

mv does its best to preserve metadata (permissions + owner/group information) when moving files around. But when the destination is not on the same filesystem than the source, this sometimes becomes impossible for reasons that are beyond mv (destination filesystem type, mount options, local vs remote user accounts, ...)

this is caused by the context, this is not a bug / limitation of mv
there is no command flag / hack / workaround
to move stuff to a different filesystem, use cp + rm instead

Create a safe temporary file or directory and display its name :

mktemp [options] template

template must contain at least 3 consecutive X in last component.

What is a symlink exploit ?

If an attacker knows in advance the name of a temporary file —e.g. has a script that creates /tmp/tempFileForRoot— he can create a symlink with that name beforehand :

/tmp/tempFileForRoot -> any/other/file

so that, while writes into /tmp/tempFileForRoot, any/other/file is actually overwritten with privileges.
This can be used to break / mess up things and workaround security measures.

Flag	Usage
-d --directory	create a directory, not a file
-u --dry-run	do not create anything, just print the name of the would-be-created file/directory This option is marked as unsafe because it leaves the possibility of a symlink exploit : in the time lapse between when the name of the temporary file/directory is displayed when this temporary file/directory is actually created an attacker could create a symlink named like this temporary file/directory. Workaround : make a temporary directory with -d, and use names of your choice within that directory (source).
--suff=suffix	append suffix to template. The same happens if template does not end with an X
-p dir --tmpdir=dir	create the temporary file into dir If dir is omitted, use $TMPDIR instead If $TMPDIR is not set, use /tmp Depending on the context, it _may_ be more appropriate to create temporary files into a RAM disk. This option doesn't exist in Bash 3.x. Use mktemp /path/to/dir/tmpFile.XXXX instead.

Create a temporary file in ...	Command	Creates	Notes
the current directory	mktemp tmpFile.XXXX	./tmpFile.4zxA	tmpFile=$(mktemp XXXXXXXX.tmp) use temp file rm "$tmpFile"
any specified directory	mktemp --tmpdir=./testDir/ test_File_XXXXXXXXXXX	./testDir/test_File_XTRr0VPklFg
the "temporary directory"	mktemp --tmpdir thisIsATempFile.XXXXXXXX mktemp	/tmp/thisIsATempFile.9vEZbi0t /tmp/tmp.fHBAF5mFbN	$TMPDIR if set /tmp otherwise
the "temporary directory", by a script	tmpFile=$(mktemp --tmpdir theNameOfMyTempFile.XXXXXXXX) tmpFile=$(mktemp --tmpdir $(basename $0).XXXXXXXXXXXXXXXXX) tmpFile=$(mktemp --tmpdir='/run/shm' $(basename $0).tmp.XXXXXXXXXXXX)	/tmp/theNameOfMyTempFile.Ts5yP2F6 /tmp/scriptName.sh.KXfKK7mYN0hFMSQwz /run/shm/scriptName.tmp.eBpsQUZCrDO4	$TMPDIR if set /tmp otherwise

mtr (My traceroute, originally called Matt's traceroute) is a computer program which combines the functionality of the traceroute and ping programs in a single network diagnostic tool.

mtr probes routers on the route path by limiting the number of hops individual packets may traverse, and listening to responses of their expiry. It will regularly repeat this process, usually once per second, and keep track of the response times of the hops along the path.

Send or read emails from the shell.

Flag	Usage
-s	specify mail subject

Send a simple mail (source) :

mail -s "subject" recipient@domain.com
When sending a mail to a localhost user, specifying recipient@domain.com, recipient@localhost or just recipient has the same result.
If you didn't enter the mail subject in the command line, type it now after the prompt : Subject: Enlarge your tennis

Type mail body :

Roses are #FF0000
Violets are #0000FF
All of my base
Are belong to you

Terminate the mail body with : . then , or with CTRL-d.
At the Cc: prompt, just hit CTRL-d again.

When mailing a localhost recipient, the /var/mail/recipient file must exist and be writable by the sender.

Send the content of a file as the mail body :

cat file | mail -s 'subject' recipient@domain.com
mail -s 'subject' recipient@domain.com < file

Be notified of a new mail in the terminal :

To receive "New mail" notifications in the terminal, you just have to set some variables in ~/.bashrc :

MAILPATH=/var/mail/bob
MAILCHECK=60

For details on MAILCHECK : man bash, then search : / MAILCHECK

make a filesystem

The Internetz are full of discussions like : What is the best filesystem ?

There is no such best filesystem because it all depends on
- your needs : type+size of data, performance, reliability, scalability, ...
- how much you're willing to invest in administration : learning curve, housekeeping tasks, ...
Moreover, since the Linux kernel itself (having highly-specific-filesystem-with-killer-feature support built-in... or not) is constantly evolving, any point of view can prove wrong a couple of months or years later.

See Comparison of file systems.

Flag	Usage
-b block-size	Specify the size of blocks in bytes. Valid block-size values are 1024, 2048 and 4096 bytes per block. If omitted, the block size will be determined from the filesystem size and usage (see -T) mkfs.ext4 only
-I	set the Inode size IFS Drive doesn't support inodes larger than 128 bytes (source)
-L	set the volume Label
-m	Specify the percentage of the filesystem blocks reserved for the super-user (default is 5%)
-T usageType	Specify how the filesystem will be used so that optimal parameters can be applied. Supported usageType values are listed in /etc/mke2fs.conf

EXT4

mkfs.ext4 -I 128 -L ConPow -m 0 /dev/ConPow
mkfs.ext4 -L ConPow -m 0 -T ext4 /dev/sde1

FAT32

mkfs.vfat -L ConPow /dev/ConPow

This command is not verbose, but don't worry

man

Usage

Flags

Example

Hack allowing to refer to any location within a manpage (source) :

modprobe

Usage

Flags

md5sum

Usage

Example

Check file integrity :

Get the MD5 hash of an arbitrary string from the command line :

Single line string :

Multiple line string :

mkdir

Usage

Flags

mv

Usage

Flags

Example

mv errors :

cannot move thisDirectory : Device or resource busy :

mv: failed to preserve ownership for 'someFile': Permission denied (source) :

mktemp

Usage

What is a symlink exploit ?

Flags

Example

mtr

Usage

mail

Usage

Flags

Example

Send a simple mail (source) :

Send the content of a file as the mail body :

Be notified of a new mail in the terminal :

mkfs

Usage

Flags

Example

EXT4

FAT32