Bash Index : D - Httqm's Docs

low level logical volume management : dmsetup manages logical devices that use the device-mapper driver.
Usage :

dmsetup command [flags] device

dirname [options] name

removes the "last part" of name and outputs its parent directory

name can be either a file or a directory :

dirname /etc/fstab
/etc
dirname /usr/bin/
/usr

dirname works on strings, not files and actually doesn't care whether name exists or not :

dirname /path/to/myFile
/path/to
dirname works/with/a/relative/path/too
works/with/a/relative/path

you can't pipe into dirname :

echo /path/to/myFile | dirname
dirname: missing operand
Try 'dirname --help' for more information.

workaround :

echo -e '/a/b/c\n/d/e/f' | xargs -I {} dirname {}
/a/b
/d/e

remove jobs from the job table
or mark jobs so that a SIGHUP signal is not sent to them if the parent shell receives it (e.g. if the user logs out or closes a terminal window)

To do so :

disown jobspec
disown PID

source : help disown

deborphan is not a Bash command, but a Debian-specific utility. It can be installed via the deborphan package.

deborphan looks for orphaned packages, that is packages which are not required by any other package upon your system.

Flag	Usage
(none)	only list packages which are in the libraries section of the Debian archive, as these are the most likely candidates for removal
-s --show-section	show the sections the packages are in
-z --show-size	show the installed size in KiB of the listed packages
--guess-data	show library (default) + data orphaned packages
--guess-all	show all orphaned packages

There are many --guess-* commands available. See the GUESSING section of man deborphan.

List orphaned packages :

deborphan

libssl1.0.0
libgnome2-bin
liblognorm1
libpsl0
libprocps3
libboost-iostreams1.55.0

deborphan --guess-all -sz

	111 main/net			libxtables10
	183 main/python			python3-cffi-backend
	352 main/java			libservlet2.5-java
	 53 main/perl			libclass-isa-perl
	458 main/admin			software-properties-common

This doesn't require root privileges since it just reads the installed packages lists.

Remove orphaned packages :

As root :

deborphan | xargs apt-get purge

Commands below have the -y flag which will uninstall packages with no prompt (but there should be no risk as apt-get is smart enough to avoid shooting its own foot ).

apt-get -y purge $(deborphan)
apt-get -y purge $(deborphan --guess-data)

dmesg's main usage is to display messages from the kernel ring buffer. It lists events in the form :

[timestamp] message

where timestamp is the number of seconds.nanoseconds since the system booted.

Information reported by dmesg is also available in /var/log/kern.log

The kernel ring buffer :

The kernel ring buffer is a memory area containing kernel messages. It's a fixed-size FIFO area.

Flag	Usage
-c --read-clear	print all messages then clear the buffer
-T --ctime	Print human-readable Timestamps Be aware that the timestamp could be inaccurate!
-w --follow	wait for new messages
-W --follow-new	Wait and print only new messages

declare (or its synonym typeset) is used to change properties of variables

Flag	Usage
-a myVariable	declare myVariable is an indexed array (example)
-A myVariable	declare myVariable is an associative array (example)
-f	list existing Bash functions (names + code)
-F	list existing Bash functions (names only)
-i myVariable	declare myVariable is an integer
-r myVariable	declare myVariable is read-only

Both are valid :

declare + assign at once :
```
declare - myVariable=value
```
declare then assign :
```
declare - myVariable
myVariable=value
```

Declare a variable as readonly :

declare -r a=1; echo $a; a=2

1
-bash: a: readonly variable

Or : readonly b=2; echo $b; b=1

2
-bash: b: readonly variable

readonly variables exist until the end of the shell process.

How to unset a readonly variable ? (source)

readonly a=1; echo $a

unset a

bash: unset: a: cannot unset: readonly variable

cat << EOF | gdb > attach $$ > call unbind_variable("a") > detach > EOF

GNU gdb (GDB) 7.4.1-debian
Copyright (C) 2012 Free Software Foundation, Inc.

(gdb) Attaching to process 29720
Reading symbols from /bin/bash...(no debugging symbols found)...done.

Loaded symbols for /lib/x86_64-linux-gnu/libnss_files.so.2
0x00007fd61bed006e in waitpid () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) $1 = 0
(gdb) Detaching from program: /bin/bash, process 29720
(gdb) quit

echo $a

(nothing)

What's the difference between declare and readonly ? (source)

The variable scope :

declare makes it local
readonly makes it global

As of bash 4.2, declare -gr seems to be identical to readonly.

Report disk free space

You may face weird situations where

deleting a file doesn't seem to free any storage space
df reports 100% disk usage whereas du can't detect where storage is actually used

Flag	Usage
-h --human-readable	print sizes in human readable format (e.g. 1K, 234M, 2G)
-i --inodes	report inode usage instead of block usage
-P --portability	use the POSIX output format : no nice tabular display anymore a single line per filesystem, even though the filesystem name is long To be used in a df -P \| grep context.
-T --print-type	print filesystem type This can be used to determine which filesystem a file belongs to : df -T $HOME Filesystem Type 1K-blocks Used Available Use% Mounted on /dev/mapper/caramba--vg-home ext4 25066604 4086168 19684052 18% /home

Check filesystems for disk full/almost full :

When above 90% of used disk space, there may be No space left on device to create a new BIG file there
There may be no inodes left, even though some (kilo|mega|giga|tera)bytes are available

Check this with :

echo 'Disk usage :'; df -Ph | grep -E '(9.|100)%'; echo 'Inodes :'; df -Phi | grep '100%'

Alternate syntax :

This allows specifying a threshold much more easily :

df -Ph | tr -d '%' | awk '$5 > 90 { print $0 }'

Find usage percent of filesystem holding /path/to/myFile :

df is actually smart enough to determine which filesystem holds /path/to/myFile and report the usage :

df -h "$HOME/.bashrc"

Filesystem			Size	Used	Avail	Use%	Mounted on
/dev/mapper/caramba--vg-home	24G	3.9G	19G	18%	/home

One-liner for scripts :

df -h "$HOME/.bashrc" | awk '$6 ~ "^/" { print $5 }'

18%

dig : DNS lookup utility

Forward DNS lookup (name into IP) :

dig name
dig name recordType
dig @dnsServer name

Reverse DNS lookup (IP to name) :

dig -x ip.add.re.ss

Investigate DNS lookup time :

#!/usr/bin/env bash

logFile=$0.csv
hostName='www.example.com'
serversList='DNS servers list'
echo "DNS Server;Time [ms]" > $logFile
for dnsServer in $serversList; do
	echo -n .
	echo -n "$dnsServer;" >> $logFile
	dig @$dnsServer $hostName | grep 'Query time' | cut -d ' ' -f 4 >> $logFile
done

Read TXT records of a domain :

dig google.com txt

Get a short response (IP only) :

Instead of dig www.example.com, which is rather verbose, get only the IP address using the +short flag : dig +short www.example.com

dd stands for data duplicator (source) but there is no consensus on the meaning of dd...

Flag	Usage
if=file	input file. This can be : /dev/zero : for an output file filled with zeros /dev/random or /dev/urandom : for an output file filled with random bytes
of=file	output file
bs=n[unit]	block size n : n bytes nkB : n1000 bytes nK : n1024 bytes nMB : n10001000 bytes nM : n10241024 bytes nGB : n100010001000 bytes nG : n102410241024 bytes bs=64k only makes the transfer go faster because dd will be reading blocks of 64k each instead of the default block size (source).
count=n	generate n blocks of the specified size
iflag=FLAG1,FLAG2,FLAGn	apply the specified flags to the read (input) file
oflag=FLAG1,FLAG2,FLAGn	apply the specified flags to the written (output) file
seek	offset
status=level	the level of information to print to stderr : none : suppresses everything but error messages (mimics a quiet mode) noxfer : suppresses the final transfer statistics progress : shows periodic transfer statistics

Flags :

direct : use direct I/O for data

How to create a test file having a specific size ?

For 100MB of 0s :: dd if=/dev/zero bs=1M count=100 of=some/file
There is a faster method (limited to ext4, btrfs, xfs and ocfs2) : fallocate. This command actually preallocates blocks to a file but doesn't write the whole file, which will look empty. Suffices like k, m, g and t stand for KiB, MiB, GiB and TiB respectively.: fallocate -l 100m some/file

Flag	Usage
-b myPackage --build myPackage	build the myPackage.deb package (details)
-c packageName --contents packageName	show contents of packageName packageName has to be available on the local filesystem
-i packageName	install the package packageName Before actually installing, check dependencies are met with (source) : dpkg --dry-run -i packageName packageName has to be available on the local filesystem
-I packageName --info packageName	show information about packageName packageName has to be available on the local filesystem
-l	list all the installed packages details on status codes Check whether packageName is installed instead of dpkg -l \| grep pattern, consider dpkg-query -l pattern
-L packageName	List files installed by the package packageName
-P packageName	Purge the package packageName : completely uninstall the package including binaries and configuration files
-r packageName	remove the package packageName, leaving the configuration files untouched.
-s packageName --status packageName	display status information about the package packageName. (this actually reads the package control file) list packages on which packageName depends
-S someFile --search someFile	display which package installed someFile. Check it : directory='/usr/bin'; nbFiles=$(ls -1 "$directory" \| wc -l); randomNumber=$(shuf -i 1-$nbFiles -n 1); randomFile=$(ls -1 "$directory" \| sed -n "$randomNumber p"); echo "Random file : '$directory/$randomFile'"; dpkgSResult=$(dpkg -S "$directory/$randomFile"); echo "... is from package : '$dpkgSResult'"; packageName=$(echo "$dpkgSResult" \| cut -d':' -f1); echo "... which also installed : "; dpkg -L "$packageName" \| grep "$directory" If this fails saying :dpkg-query: no path found matching pattern someFile, it may be because someFile is a symlink.

dpkg -l status codes (details) :

1^st character : expected status of the package
- h : hold (i.e. won't be upgraded by apt upgrade. How to list held packages.)
- i : installed
- p : purged
- r : removed
- u : unknown
2^nd character : current status of the package
- F : halF configured (configuration failed for some reason)
- H : Half installed (installation failed for some reason)
- U : Unpacked
- W : triggers aWaiting (package is waiting for a trigger from another package)
- c : configuration files are still present in the system
  See apt-get purge
- i : installed
- n : not installed
- t : triggers pending (package has been triggered)
3^rd character : error flag
- : (empty) no error
- R : Reinstall required (package broken, reinstallation required)

Common codes :

hi : installed and will stay in the current version (because of hold)
ii : package is installed and everything is going extremely well
rc : package has been removed (i.e. cleanly uninstalled) but its configuration files are still there
un : unknown + not installed, a package that has never been installed on this system

List of all existing codes :

(source) dpkg -l | head -3
man dpkg-query

List dependencies of packageName (see also aptitude why) :

The packages packageName depends on :

dpkg -s packageName | grep "^Depends"
apt-cache show mysql-server | grep Depends
or even better : apt-cache depends mysql-server

Depends: mysql-server-5.5

The packages that depend on packageName (source) :

apt-cache rdepends mysql-server

mysql-server
Reverse Depends:
  mysql-server-5.5
 |yubikey-val		meaning of this | ? Read below.
 |yubikey-ksm
 |spip
 |prometheus-mysqld-exporter
  openstack-cloud-services

What does the | in the output of apt-cache rdepends packageName mean ? (source) :

As seen above, apt-cache rdepends packageName lists the packages that depend on packageName :

apt-cache rdepends mutt

mutt
Reverse Depends:
  t-prot
  notmuch-mutt
  devscripts
  urlview
  urlscan

 |sylph-searcher	sylph-searcher depends on mutt or an alternate package (shown by the |)

  mixmaster
 |metche
  lbdb

  automysqlbackup	automysqlbackup depends on mutt, and there's no alternative

the listed packages have some kind of dependency on mutt (Depends, Recommends, Suggests, Enhances, ...)

check it : apt-cache show sylph-searcher metche automysqlbackup | grep -E '^Package|mutt'

Package: sylph-searcher
Suggests: sylpheed | claws-mail | mutt | wl	one of these will suffice

Package: metche
Depends: debconf (>= 0.5) | debconf-2.0, mutt | mailx | mail-transport-agent, bzip2, ucf	debconf AND (mutt OR mailx OR mail-transport-agent) AND bzip2 AND ucf

Package: automysqlbackup
Recommends: mutt	no alternative

Check whether packageName is installed :

dpkg -l | grep packageName
dpkg -l packageName &>/dev/null && echo 'INSTALLED' || echo 'NOT INSTALLED'

DMI table decoder. To extract the whole tables, just run dmidecode as root.
The -t (or --type) flag displays the available tables :

bios
system
baseboard
chassis
processor
memory
cache
connector
slot

Then, to get details about a specific component : dmidecode -t processor.
It is also possible to query tables numerically (codes available with man dmidecode). Information about the motherboard (2) and on-board devices (10) : dmidecode -t 2,10

Estimate disk space usage

You may face weird situations where

deleting a file doesn't seem to free any storage space
df reports 100% disk usage whereas du can't detect where storage is actually used

Flag	Usage
--apparent-size	print apparent sizes, rather than disk usage; although the apparent size is usually smaller, it may be larger due to holes in ('sparse') files, internal fragmentation, indirect blocks, and the like
-c	display a grand total
--max-depth depth (Linux) -d depth (*BSD ?)	display an entry for all files and directories depth directories deep max-depth conflicts with s
-h	display results in human readable format
-m	display file size in megabytes
-S --separate-dirs	ignore subdirectory sizeThis REALLY ignores subdirectories !
-s	summarize : display only a total for each argument s conflicts with max-depth
-x	only consider files that are on the same filesystem (=device) than the file given as parameter : du -x /. This prevents going into /mnt/... filesystems.

Size of a directory :

du -sh /path/to/directory/

Size of children directories :

commands below rely on that sort is smart enough to sort human units

generic (lists directories only) :: du -h . --max-depth=1 | sort -h r | head -20
alternate (also lists files) :: The specified path must end with /*

du -xhs ./* | sort -h r

Size of files only contained in children directories (ignoring deeper directories) :

du -S -sh * | sort -nr

Find which directory is eating megabytes on the / filesystem :

du -mx / | sort -nr | less

After copying data from a server to another, the reported "disk usages" are different. Why ? (source : 1, 2, 3)

Keep in mind that :

du is about the disk usage. And disk usage != sum of file sizes.
du reports estimates (for byte-count precision of disk usage, consider --apparent-size)

It is entirely possible that the same set of files use different amount of disk space. This is because of the intricacies of the file system (including block size, internal fragmentation, ...) (source, details).

dmsetup

dirname

disown

Usage

deborphan

Usage

Flags

Example

List orphaned packages :

Remove orphaned packages :

dmesg

Usage

The kernel ring buffer :

Flags

declare

Usage

Flags

Example

Declare a variable as readonly :

How to unset a readonly variable ? (source)

What's the difference between declare and readonly ? (source)

df

Usage

Flags

Example

Check filesystems for disk full/almost full :

Alternate syntax :

Find usage percent of filesystem holding /path/to/myFile :

One-liner for scripts :

dig

Usage

Example

Investigate DNS lookup time :

Read TXT records of a domain :

Get a short response (IP only) :

dd

Usage

Flags

Flags :

Example

How to create a test file having a specific size ?

dpkg

Flags

Exit Status

dpkg -l status codes (details) :

List of all existing codes :

Example

List dependencies of packageName (see also aptitude why) :

The packages packageName depends on :

The packages that depend on packageName (source) :

What does the | in the output of apt-cache rdepends packageName mean ? (source) :

Check whether packageName is installed :

dmidecode

du

Usage

Flags

Example

Size of a directory :

Size of children directories :

Size of files only contained in children directories (ignoring deeper directories) :

Find which directory is eating megabytes on the / filesystem :

After copying data from a server to another, the reported "disk usages" are different. Why ? (source : 1, 2, 3)