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 the variable scope local, while readonly makes the variable scope 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

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

• diff --color=always -Bbiy -W $(tput cols) file1 file2 | less -R

• fileA='inventory_CW_STG_PCK.ini'
  fileB='inventory_FE_BCR.ini'
  diff --color=always --suppress-common-lines -Bbiy -W $(tput cols) "$fileA" "$fileB" | less -R

  All lines output with this command syntax have differences, even if they are not color-highlighted.

#!/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'

The --date='dateString' is a human-readable format such as next Thursday or 1 month ago. A date string may contain items indicating :

• calendar date
• time of day
• time zone
• day of the week
• relative time
• relative date
• numbers

Keywords

Examples

Unix timestamp into date (source) :

for unixTime in 1234567890 1500000000 1600000000 2000000000; do echo -e "UNIX time : $unixTime\t==>\tLOCAL TIME : $(date -d@$unixTime)"; done

UNIX time : 1234567890	==>	LOCAL TIME : Sat Feb 14 00:31:30 CET 2009
UNIX time : 1500000000	==>	LOCAL TIME : Fri Jul 14 04:40:00 CEST 2017
UNIX time : 1600000000	==>	LOCAL TIME : Sun Sep 13 14:26:40 CEST 2020
UNIX time : 2000000000	==>	LOCAL TIME : Wed May 18 05:33:20 CEST 2033

Date string YYYY-MM-DD hh:mm:ss date into Unix timestamp :

date -d"2013-12-16 00:30:01" +%s

Unix timestamp into a formatted date string :

date -d@1387215550 "+%d/%m/%Y - %H:%M"

general case :

startDate='2016-12-18 10:05:00'; endDate='today'; echo "scale=1; ("$(date --date "$endDate" +%s) - $(date --date "$startDate" +%s)") / 86400" | bc

excluding start and end days :

• startDate='yesterday'; endDate='tomorrow'; echo "scale=1; ("$(date --date "$endDate 00:00:00" +%s) - $(date --date "$startDate 23:59:59" +%s)") / 86400" | bc
• startDate='20170330'; endDate='20170401'; printf %0.f $(echo "scale=1; ("$(date --date "$endDate" +%s) - $(date --date "$startDate" +%s)") / 86400 -1" | bc)

go even further :

startDate='2019-07-06'; endDate='2019-09-02'; nbDays=$(echo "scale=1; ("$(date --date "$endDate" +%s) - $(date --date "$startDate" +%s)") / 86400" | bc); halfNbDays=$(echo "scale=1; $nbDays/2" | bc); middleDay=$(date +"%F (%a)" --date "$startDate +$(echo $halfNbDays | cut -d'.' -f1) days"); echo -e "Nb days =|$nbDays\nHalf =|$halfNbDays\nMiddle day =|$middleDay" | column -s '|' -t

There's a hack to compute dates with integers only, which makes the whole command not exact if there's an odd number of days .

• I am convinced it would be possible to do this with date + Bash (believe me, I like scripting ! ), but in the light of getting-results-fast, LibreOffice Calc's workday function wins !
• For those wondering, Excel has a similar workday function.

startYear='2014'
startMonth='08'
startDay='06'

startHour='13'
startMinute='00'
startSecond='00'

nbDaysBack=4
resolutionInHours=1	# number of hours between 2 generated datetimes

#dateTimeFormat='%d%m%y-%H'	# ddmmyy-hh
dateTimeFormat='%Y-%m-%d %H'	# yyyy-mm-dd hh

initialTimestamp=$(date -d"$startYear-$startMonth-$startDay $startHour:$startMinute:$startSecond" +%s)
nbLoops=$((24/$resolutionInHours*$nbDaysBack))
timestamp=$initialTimestamp
for i in $(seq 1 $nbLoops);do
	formattedDate=$(date -d@$timestamp +"$dateTimeFormat")
	echo $formattedDate

	# ... now do something smart with the formatted date !

	timestamp=$((timestamp-(3600*$resolutionInHours)))
done

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)