Bash Index : D - The 'D' Bash commands : description, flags and examples

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
	

As root :

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

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

1
-bash: a: readonly variable

2
-bash: b: readonly variable

readonly variables exist until the end of the shell process.

readonly a=1; echo $a

1

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)

• declare makes it local
• readonly makes it global

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

• 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

Alternate syntax :

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

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

One-liner for scripts :

18%

#!/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

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

rc

r : the package was marked for removal
c : the configuration files are currently present in the system

See apt-get purge

ii

installed

List of all existing codes :

The packages packageName depends on :

Depends: mysql-server-5.5

The packages that depend on packageName (source) :

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) :

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

unsorted :

du -h --max-depth=1 /path/to/directory/

sorted from biggest to smallest :

• du -m . --max-depth=1 | sort -nr
• du -m . --max-depth=1 | sort -nr | head -20

sorted from biggest to smallest (alternate) :

The specified path must end with /*

du -xms ./* | sort -nr

And since sort is smart enough to sort human units, we can also run :

du -xhs ./* | sort -rh

• 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)