Articles taggués ‘bash’

30 Handy Bash Shell Aliases For Linux / Unix / Mac OS X

28/07/2017 Comments off

An alias is nothing but the shortcut to commands. The alias command allows the user to launch any command or group of commands (including options and filenames) by entering a single word. Use alias command to display a list of all defined aliases. You can add user-defined aliases to ~/.bashrcfile. You can cut down typing time with these aliases, work smartly, and increase productivity at the command prompt.

More about aliases

The general syntax for the alias command for the bash shell is as follows:

Task: List aliases

Type the following command:


Sample outputs:

alias ..='cd ..'
alias amazonbackup='s3backup'
alias apt-get='sudo apt-get'

By default alias command shows a list of aliases that are defined for the current user.

Task: Define / create an alias (bash syntax)

To create the alias use the following syntax:

alias name=value
alias name='command'
alias name='command arg1 arg2'
alias name='/path/to/script'
alias name='/path/to/ arg1'

In this example, create the alias c for the commonly used clear command, which clears the screen, by typing the following command and then pressing the ENTER key:

alias c='clear'

Then, to clear the screen, instead of typing clear, you would only have to type the letter ‘c’ and press the [ENTER] key:


Task: Disable an alias temporarily (bash syntax)

An alias can be disabled temporarily using the following syntax:

## path/to/full/command
## call alias with a backslash ##

Task: Remove an alias (bash syntax)

You need to use the command called unalias to remove aliases. Its syntax is as follows:

unalias aliasname

In this example, remove the alias c which was created in an earlier example:

unalias c

You also need to delete the alias from the ~/.bashrc file using a text editor (see next section).

Task: Make aliases permanent (bash syntax)

The alias c remains in effect only during the current login session. Once you logs out or reboot the system the alias c will be gone. To avoid this problem, add alias to your ~/.bashrc file, enter:

vi ~/.bashrc

The alias c for the current user can be made permanent by entering the following line:

alias c='clear'

Save and close the file. System-wide aliases (i.e. aliases for all users) can be put in the /etc/bashrc file. Please note that the alias command is built into a various shells including ksh, tcsh/csh, ash, bash and others.

A note about privileged access

You can add code as follows in ~/.bashrc:

# if user is not root, pass all commands via sudo #
if [ $UID -ne 0 ]; then
    alias reboot='sudo reboot'
    alias update='sudo apt-get upgrade'

A note about os specific aliases

You can add code as follows in ~/.bashrc using the case statement:

### Get os name via uname ###
### add alias as per os using $_myos ###
case $_myos in
   Linux) alias foo='/path/to/linux/bin/foo';;
   FreeBSD|OpenBSD) alias foo='/path/to/bsd/bin/foo' ;;
   SunOS) alias foo='/path/to/sunos/bin/foo' ;;
   *) ;;

30 uses for aliases

You can define various types aliases as follows to save time and increase productivity.

#1: Control ls command output

The ls command lists directory contents and you can colorize the output:

## Colorize the ls output ##
alias ls='ls --color=auto'
## Use a long listing format ##
alias ll='ls -la'
## Show hidden files ##
alias l.='ls -d .* --color=auto'

#2: Control cd command behavior

## get rid of command not found ##
alias cd..='cd ..'
## a quick way to get out of current directory ##
alias ..='cd ..'
alias ...='cd ../../../'
alias ....='cd ../../../../'
alias .....='cd ../../../../'
alias .4='cd ../../../../'
alias .5='cd ../../../../..'

#3: Control grep command output

grep command is a command-line utility for searching plain-text files for lines matching a regular expression:

## Colorize the grep command output for ease of use (good for log files)##
alias grep='grep --color=auto'
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'

#4: Start calculator with math support

alias bc='bc -l'

#4: Generate sha1 digest

alias sha1='openssl sha1'

#5: Create parent directories on demand

mkdir command is used to create a directory:

alias mkdir='mkdir -pv'

#6: Colorize diff output

You can compare files line by line using diff and use a tool called colordiff to colorize diff output:

# install colordiff package 🙂
alias diff='colordiff'

#7: Make mount command output pretty and human readable format

alias mount='mount |column -t'

#8: Command short cuts to save time

# handy short cuts #
alias h='history'
alias j='jobs -l'

#9: Create a new set of commands

alias path='echo -e ${PATH//:/\\n}'
alias now='date +"%T"'
alias nowtime=now
alias nowdate='date +"%d-%m-%Y"'

#10: Set vim as default

alias vi=vim
alias svi='sudo vi'
alias vis='vim "+set si"'
alias edit='vim'

#11: Control output of networking tool called ping

# Stop after sending count ECHO_REQUEST packets #
alias ping='ping -c 5'
# Do not wait interval 1 second, go fast #
alias fastping='ping -c 100 -s.2'

#12: Show open ports

Use netstat command to quickly list all TCP/UDP port on the server:

alias ports='netstat -tulanp'

#13: Wakeup sleeping servers

Wake-on-LAN (WOL) is an Ethernet networking standard that allows a server to be turned on by a network message. You can quickly wakeup nas devices and server using the following aliases:

## replace mac with your actual server mac address #
alias wakeupnas01='/usr/bin/wakeonlan 00:11:32:11:15:FC'
alias wakeupnas02='/usr/bin/wakeonlan 00:11:32:11:15:FD'
alias wakeupnas03='/usr/bin/wakeonlan 00:11:32:11:15:FE'

#14: Control firewall (iptables) output

Netfilter is a host-based firewall for Linux operating systems. It is included as part of the Linux distribution and it is activated by default. This post list most common iptables solutions required by a new Linux user to secure his or her Linux operating system from intruders.

## shortcut for iptables and pass it via sudo#
alias ipt='sudo /sbin/iptables'
# display all rules #
alias iptlist='sudo /sbin/iptables -L -n -v --line-numbers'
alias iptlistin='sudo /sbin/iptables -L INPUT -n -v --line-numbers'
alias iptlistout='sudo /sbin/iptables -L OUTPUT -n -v --line-numbers'
alias iptlistfw='sudo /sbin/iptables -L FORWARD -n -v --line-numbers'
alias firewall=iptlist

#15: Debug web server / cdn problems with curl

# get web server headers #
alias header='curl -I'
# find out if remote server supports gzip / mod_deflate or not #
alias headerc='curl -I --compress'

#16: Add safety nets

# do not delete / or prompt if deleting more than 3 files at a time #
alias rm='rm -I --preserve-root'
# confirmation #
alias mv='mv -i'
alias cp='cp -i'
alias ln='ln -i'
# Parenting changing perms on / #
alias chown='chown --preserve-root'
alias chmod='chmod --preserve-root'
alias chgrp='chgrp --preserve-root'

#17: Update Debian Linux server

apt-get command is used for installing packages over the internet (ftp or http). You can also upgrade all packages in a single operations:

# distro specific - Debian / Ubuntu and friends #
# install with apt-get
alias apt-get="sudo apt-get"
alias updatey="sudo apt-get --yes"
# update on one command
alias update='sudo apt-get update && sudo apt-get upgrade'

#18: Update RHEL / CentOS / Fedora Linux server

yum command is a package management tool for RHEL / CentOS / Fedora Linux and friends:

## distrp specifc RHEL/CentOS ##
alias update='yum update'
alias updatey='yum -y update'

#19: Tune sudo and su

# become root #
alias root='sudo -i'
alias su='sudo -i'

#20: Pass halt/reboot via sudo

shutdown command bring the Linux / Unix system down:

# reboot / halt / poweroff
alias reboot='sudo /sbin/reboot'
alias poweroff='sudo /sbin/poweroff'
alias halt='sudo /sbin/halt'
alias shutdown='sudo /sbin/shutdown'

#21: Control web servers

# also pass it via sudo so whoever is admin can reload it without calling you #
alias nginxreload='sudo /usr/local/nginx/sbin/nginx -s reload'
alias nginxtest='sudo /usr/local/nginx/sbin/nginx -t'
alias lightyload='sudo /etc/init.d/lighttpd reload'
alias lightytest='sudo /usr/sbin/lighttpd -f /etc/lighttpd/lighttpd.conf -t'
alias httpdreload='sudo /usr/sbin/apachectl -k graceful'
alias httpdtest='sudo /usr/sbin/apachectl -t && /usr/sbin/apachectl -t -D DUMP_VHOSTS'

#22: Alias into our backup stuff

# if cron fails or if you want backup on demand just run these commands #
# again pass it via sudo so whoever is in admin group can start the job #
# Backup scripts #
alias backup='sudo /home/scripts/admin/scripts/backup/ --type local --taget /raid1/backups'
alias nasbackup='sudo /home/scripts/admin/scripts/backup/ --type nas --target nas01'
alias s3backup='sudo /home/scripts/admin/scripts/backup/ --type nas --target nas01 --auth /home/scripts/admin/.authdata/amazon.keys'
alias rsnapshothourly='sudo /home/scripts/admin/scripts/backup/ --type remote --target nas03 --auth /home/scripts/admin/.authdata/ssh.keys --config /home/scripts/admin/scripts/backup/config/adsl.conf'
alias rsnapshotdaily='sudo /home/scripts/admin/scripts/backup/ --type remote --target nas03 --auth /home/scripts/admin/.authdata/ssh.keys --config /home/scripts/admin/scripts/backup/config/adsl.conf'
alias rsnapshotweekly='sudo /home/scripts/admin/scripts/backup/ --type remote --target nas03 --auth /home/scripts/admin/.authdata/ssh.keys --config /home/scripts/admin/scripts/backup/config/adsl.conf'
alias rsnapshotmonthly='sudo /home/scripts/admin/scripts/backup/ --type remote --target nas03 --auth /home/scripts/admin/.authdata/ssh.keys --config /home/scripts/admin/scripts/backup/config/adsl.conf'
alias amazonbackup=s3backup

#23: Desktop specific – play avi/mp3 files on demand

## play video files in a current directory ##
# cd ~/Download/movie-name
# playavi or vlc
alias playavi='mplayer *.avi'
alias vlc='vlc *.avi'
# play all music files from the current directory #
alias playwave='for i in *.wav; do mplayer "$i"; done'
alias playogg='for i in *.ogg; do mplayer "$i"; done'
alias playmp3='for i in *.mp3; do mplayer "$i"; done'
# play files from nas devices #
alias nplaywave='for i in /nas/multimedia/wave/*.wav; do mplayer "$i"; done'
alias nplayogg='for i in /nas/multimedia/ogg/*.ogg; do mplayer "$i"; done'
alias nplaymp3='for i in /nas/multimedia/mp3/*.mp3; do mplayer "$i"; done'
# shuffle mp3/ogg etc by default #
alias music='mplayer --shuffle *'

#24: Set default interfaces for sys admin related commands

vnstat is console-based network traffic monitor. dnstop is console tool to analyze DNS traffic. tcptrack and iftop commands displays information about TCP/UDP connections it sees on a network interface and display bandwidth usage on an interface by host respectively.

## All of our servers eth1 is connected to the Internets via vlan / router etc ##
alias dnstop='dnstop -l 5 eth1'
alias vnstat='vnstat -i eth1'
alias iftop='iftop -i eth1'
alias tcpdump='tcpdump -i eth1'
alias ethtool='ethtool eth1'
# work on wlan0 by default #
# Only useful for laptop as all servers are without wireless interface
alias iwconfig='iwconfig wlan0'

#25: Get system memory, cpu usage, and gpu memory info quickly

## pass options to free ##
alias meminfo='free -m -l -t'
## get top process eating memory
alias psmem='ps auxf | sort -nr -k 4'
alias psmem10='ps auxf | sort -nr -k 4 | head -10'
## get top process eating cpu ##
alias pscpu='ps auxf | sort -nr -k 3'
alias pscpu10='ps auxf | sort -nr -k 3 | head -10'
## Get server cpu info ##
alias cpuinfo='lscpu'
## older system use /proc/cpuinfo ##
##alias cpuinfo='less /proc/cpuinfo' ##
## get GPU ram on desktop / laptop##
alias gpumeminfo='grep -i --color memory /var/log/Xorg.0.log'

#26: Control Home Router

The curl command can be used to reboot Linksys routers.

# Reboot my home Linksys WAG160N / WAG54 / WAG320 / WAG120N Router / Gateway from *nix.
alias rebootlinksys="curl -u 'admin:my-super-password' ''"
# Reboot tomato based Asus NT16 wireless bridge
alias reboottomato="ssh admin@ /sbin/reboot"

#27 Resume wget by default

The GNU Wget is a free utility for non-interactive download of files from the Web. It supports HTTP, HTTPS, and FTP protocols, and it can resume downloads too:

## this one saved by butt so many times ##
alias wget='wget -c'

#28 Use different browser for testing website

## this one saved by butt so many times ##
alias ff4='/opt/firefox4/firefox'
alias ff13='/opt/firefox13/firefox'
alias chrome='/opt/google/chrome/chrome'
alias opera='/opt/opera/opera'
#default ff
alias ff=ff13
#my default browser
alias browser=chrome

#29: A note about ssh alias

Do not create ssh alias, instead use ~/.ssh/config OpenSSH SSH client configuration files. It offers more option. An example:

Host server10
  IdentityFile ~/backups/.ssh/id_dsa
  user foobar
  Port 30000
  ForwardX11Trusted yes
  TCPKeepAlive yes

You can now connect to peer1 using the following syntax:
$ ssh server10

#30: It’s your turn to share…

## set some other defaults ##
alias df='df -H'
alias du='du -ch'
# top is atop, just like vi is vim
alias top='atop'
## nfsrestart - must be root ##
## refresh nfs mount / cache etc for Apache ##
alias nfsrestart='sync && sleep 2 && /etc/init.d/httpd stop && umount netapp2:/exports/http && sleep 2 && mount -o rw,sync,rsize=32768,wsize=32768,intr,hard,proto=tcp,fsc natapp2:/exports /http/var/www/html && /etc/init.d/httpd start'
## Memcached server status ##
alias mcdstats='/usr/bin/memcached-tool stats'
alias mcdshow='/usr/bin/memcached-tool display'
## quickly flush out memcached server ##
alias flushmcd='echo "flush_all" | nc 11211'
## Remove assets quickly from Akamai / Amazon cdn ##
alias cdndel='/home/scripts/admin/cdn/purge_cdn_cache --profile akamai'
alias amzcdndel='/home/scripts/admin/cdn/purge_cdn_cache --profile amazon'
## supply list of urls via file or stdin
alias cdnmdel='/home/scripts/admin/cdn/purge_cdn_cache --profile akamai --stdin'
alias amzcdnmdel='/home/scripts/admin/cdn/purge_cdn_cache --profile amazon --stdin'


This post summarizes several types of uses for *nix bash aliases:

  1. Setting default options for a command (e.g. set eth0 as default option for ethtool command via alias ethtool='ethtool eth0' ).
  2. Correcting typos (cd.. will act as cd .. via alias cd..='cd ..').
  3. Reducing the amount of typing.
  4. Setting the default path of a command that exists in several versions on a system (e.g. GNU/grep is located at /usr/local/bin/grep and Unix grep is located at /bin/grep. To use GNU grep use alias grep='/usr/local/bin/grep' ).
  5. Adding the safety nets to Unix by making commands interactive by setting default options. (e.g. rm, mv, and other commands).
  6. Compatibility by creating commands for older operating systems such as MS-DOS or other Unix like operating systems (e.g. alias del=rm ).

I’ve shared my aliases that I used over the years to reduce the need for repetitive command line typing. If you know and use any other bash/ksh/csh aliases that can reduce typing, share below in the comments.

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How to check the file size in Linux/Unix bash shell scripting

28/07/2017 Comments off


How to check file size in unix using wc command

The wc command shows the number of lines, words, and bytes contained in file. The syntax is as follows to get the file size:
wc -c /path/to/file
wc -c /etc/passwd

Sample outputs:

5253 /etc/passwd

You can easily extract the first field either using the cut or awk command:
wc -c /etc/passwd | awk '{print $1}'
Sample outputs:


OR assign this size to a bash variable:

myfilesize=$(wc -c "/etc/passwd" | awk '{print $1}')
printf "%d\n" $myfilesize
echo "$myfilesize"

How to get the size of a file in a bash script using stat command

The stat command shows information about the file. The syntax is as follows to get the file size on GNU/Linux stat:
stat -c %s "/etc/passwd"
stat --format=%s "/etc/passwd"
To assign this size to a bash variable:

myfilesize=$(stat --format=%s "/etc/passwd")
echo "$myfilesize"
## or ##
myFileSizeCheck=$(stat -c %s "/etc/resolv.conf")
printf "My file size = %d\n" $myFileSizeCheck

The syntax is as follows to get the file size on BSD/MacOS stat:
stat -f %z "/etc/passwd"
Please note that if the file is symlink you will get size of that link only with the stat command.

du command example

The syntax is

du --apparent-size --block-size=1  "/etc/passwd"
mfs=$(du --apparent-size --block-size=1  "$fileName" | awk '{ print $1}')
echo "$fileName size = ${mfs}"

Sample outputs from above commands:

Fig.01: How to check size of a file using a bash/ksh/zsh/sh/tcsh shell?Fig.01: How to check size of a file using a bash/ksh/zsh/sh/tcsh shell?


Find command example

The syntax is:

find "/etc/passwd" -printf "%s"
find "/etc/passwd" -printf "%s\n"
mysize=$(find "$fileName" -printf "%s")
printf "File %s size = %d\n" $fileName $mysize
echo "${fileName} size is ${mysize} bytes."
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How to get domain name from URL in bash shell script

27/07/2017 Comments off
How can I extract or fetch a domain name from a URL string (e.g. using bash shell scripting under Linux or Unix-like operating system?

You can use standard Unix commands such as sed, awk, grep, Perl, Python and more to get domain name from URL. No need to write regex. It is pretty simple.

Let use see various commands and option to grab the domain part from given variable under Linux or Unix-like system.

Get domain name from full URL

Say your url name is stored in a bash shell variable such as $x:
You can use the awk as follows:
echo "$x" | awk -F/ '{print $3}'
### OR ###
awk -F/ '{print $3}' <<<$x

Sample outputs:

Extract domain name from URL using sed

Here is a sample sed command:
echo "$url" | sed -e 's|^[^/]*//||' -e 's|/.*$||'

Extract domain name from URL using bash shell parameter substitution

Another option is to use bash shell parameter substitution:

# My shell variable 
## Remove protocol part of url ##
## Remove username and/or username:password part of URL ##
## Remove rest of urls ##
## Show domain name only ##
echo "$f"

Shell script example

A shell script to purge urls from Cloudflare by matching domain name part:

bon=$(tput bold)
boff=$(tput sgr0)
[ "$urls" == "" ] && { echo "Usage: $0 url"; exit 1; }
echo "Purging..."
for u in $urls
     echo -n "${bon}${c}${boff}.${u}: "
     ## Get domain name ##
     d="$(echo $u | awk -F/ '{ print $3}')"
     ## Set API_KEY, Email_ID, and ZONE_ID as per domain ##
     case $d in zone_id="ID_1"; api_key="MY_KEY_1"; email_id="";; zone_id="ID_2"; api_key="MY_KEY_2"; email_id="";;
	     *) echo "Domain not configured."; continue;;
     ## Do it ##
     curl -X DELETE "${zone_id}/purge_cache" \
     -H "X-Auth-Email: ${email_id}" \
     -H "X-Auth-Key: ${api_key}" \
     -H "Content-Type: application/json" \
     --data "{\"files\":[\"${u}\"]}"
     (( c++ ))
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How to count total number of word occurrences using grep on Linux or Unix

27/07/2017 Comments off

I want to find out how many times a word (say foo or an IP address) occurs in a text file using the grep command on Linux or Unix-like system?

You can use the grep command to search strings, words, text, and numbers for a given patterns. You can pass the -coption to grep command. It only shows the number of times that the pattern has been matched for each file.




Show the total number of times that the word foo appears in a file named bar.txt

The syntax is:
grep -c string filename
grep -c foo bar.txt

Sample outputs:


To count total number of occurrences of word in a file named /etc/passwd root using grep, run:
grep -c root /etc/passwd
To verify that run:
grep --color root /etc/passwd
Pass the -w option to grep to select only an entire word or phrase that matches the specified pattern:
grep -w root /etc/passwd
grep -c -w root /etc/passwd
In this example only match a word being with root:
grep --color -w '^root' /etc/passwd
grep -c -w '^root' /etc/passwd

To show only the matching part of the lines.
grep -o 'root' /etc/passwd
grep -c -o 'root' /etc/passwd

Sample session:

Fig.01: Counting occurrence of words/strings using grep commandFig.01: Counting occurrence of words/strings using grep command

How to display countdown timer in bash shell script running on Linux/Unix

27/07/2017 Comments off
I want to display a countdown before purging cache from CDN network. Is there an existing command to show a conuntdown from 30..1 as 30,29,28,…1 on Linux or Unix bash shell script?

There are various ways to show a countdown in your shell scripts. 

First define your message:
msg="Purging cache please wait..."
Now clear the screen and display the message at row 10 and column 5 using tput:
tput cup 10 5

Next you need to display the message:
echo -n "$msg"

Find out the length of string:
Calculate the next column:
l=$(( l+5 ))
Finally use a bash for loop to show countdown:
for i in {30..01}
tput cup 10 $l
echo -n "$i"
sleep 1

Here is a complete shell script:

# Purpose: Purge urls from Cloudflare Cache
# Author: Vivek Gite {} under GPL v2.x+
# --------------------------------------------------------
# Set me first #
countdown() {
        msg="Purging ${1}..."
        tput cup $row $col
        echo -n "$msg"
        l=$(( l+$col ))
        for i in {30..1}
                tput cup $row $l
                echo -n "$i"
                sleep 1
# Do it
for u in $urls
     curl -X DELETE "${zone_id}/purge_cache" \
     -H "X-Auth-Email: ${email_id}" \
     -H "X-Auth-Key: ${api_key}" \
     -H "Content-Type: application/json" \
     --data "{\"files\":[\"${u}\",\"${amp_url}\"]}" &>/dev/null &&  countdown "$u"

You can run it as follows:
./ url1 url2

POSIX shell version

From this post:

  set -- $*
  secs=$(( ${1#0} * 3600 + ${2#0} * 60 + ${3#0} ))
  while [ $secs -gt 0 ]
    sleep 1 &
    printf "\r%02d:%02d:%02d" $((secs/3600)) $(( (secs/60)%60)) $((secs%60))
    secs=$(( $secs - 1 ))

It can be run as follows:
countdown "00:00:10" # 10 sec
countdown "00:00:30" # 30 sec
countdown "00:01:42" # 1 min 42 sec


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Bash Shell: Replace a String With Another String In All Files Using sed and Perl -pie Options

27/07/2017 Comments off

String search and replace

How do I replace a string with another string in all files? For example, ~/foo directory has 100s of text file and I’d like to find out xyz string and replace with abc. I’d like to use sed or any other tool to replace all occurrence of the word.

The sed command is designed for this kind of work i.e. find and replace strings or words from a text file under Apple OX, *BSD, Linux, and UNIX like operating systems. The perl can be also used as described below.

sed replace word / string syntax

The syntax is as follows:
sed -i 's/old-word/new-word/g' *.txt

GNU sed command can edit files in place (makes backup if extension supplied) using the -i option. If you are using an old UNIX sed command version try the following syntax:

sed 's/old/new/g' input.txt > output.txt

You can use old sed syntax along with bash for loop:

[ ! -d $BPATH ] && mkdir -p $BPATH || :
for f in $DPATH
  if [ -f $f -a -r $f ]; then
    /bin/cp -f $f $BPATH
   sed "s/$OLD/$NEW/g" "$f" > $TFILE && mv $TFILE "$f"
   echo "Error: Cannot read $f"
/bin/rm $TFILE

A Note About Bash Escape Character

A non-quoted backslash \ is the Bash escape character. It preserves the literal value of the next character that follows, with the exception of newline. If a \newline pair appears, and the backslash itself is not quoted, the \newline is treated as a line continuation (that is, it is removed from the input stream and effectively ignored). This is useful when you would like to deal with UNIX paths. In this example, the sed command is used to replace UNIX path “/nfs/apache/logs/rawlogs/access.log” with “__DOMAIN_LOG_FILE__”:

## Our path
## Escape path for sed using bash find and replace 
# replace __DOMAIN_LOG_FILE__ in our sample.awstats.conf
sed -e "s/__DOMAIN_LOG_FILE__/${_r1}/" /nfs/conf/awstats/sample.awstats.conf  > /nfs/apache/logs/awstats/awstats.conf 
# call awstats
/usr/bin/awstats -c /nfs/apache/logs/awstats/awstats.conf

The $_r1 is escaped using bash find and replace parameter substitution syntax to replace each occurrence of / with \/.

perl -pie Syntax For Find and Replace

The syntax is as follows:
perl -pie 's/old-word/new-word/g' input.file > new.output.file

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Introduction aux scripts shell

26/07/2017 Comments off

Vous venez d’apprendre à utiliser un éditeur de texte puissant comme Vim. Cela va vous être particulièrement utile pour les chapitres à venir.

Entrons maintenant dans le vif du sujet : la programmation shell. De quoi s’agit-il ?

Imaginez un minilangage de programmation intégré à Linux. Ce n’est pas un langage aussi complet que peuvent l’être le C, le C++ ou le Java par exemple, mais cela permet d’automatiser la plupart de vos tâches : sauvegarde des données, surveillance de la charge de votre machine, etc.

On aurait très bien pu faire tout cela en créant un programme en C par exemple. Le gros avantage du langage shell est d’être totalement intégré à Linux : il n’y a rien à installer, rien à compiler. Et surtout : vous avez très peu de nouvelles choses à apprendre. En effet, toutes les commandes que l’on utilise dans les scripts shell sont des commandes du système que vous connaissez déjà : ls, cut, grep, sort…

On parlera beaucoup de shell dans cette section. De quoi s’agit-il exactement ? Nous répondrons à cette question en premier.
Ensuite, nous réaliserons notre tout premier script shell qui affiche un message à l’écran… et nous pourrons alors passer aux choses sérieuses dès le chapitre suivant !

Qu’est-ce qu’un shell ?

Dès le début, j’ai fait la distinction entre les deux environnements très différents disponibles sous Linux :

  • l’environnement console ;
  • l’environnement graphique.

La plupart du temps, sur sa machine, on a tendance à utiliser l’environnement graphique, qui est plus intuitif. Cependant, la console est aussi un allié très puissant qui permet d’effectuer des actions habituellement difficiles à réaliser dans un environnement graphique.

Je vous avais dit qu’il y avait plusieurs environnements graphiques disponibles (Unity, KDE, XFCE…) mais qu’il n’y avait qu’une seule console. J’ai menti.

Il existe plusieurs environnements console : les shells

La différence est moins tape-à-l’œil que dans le mode graphique (où l’on voit tout de suite que les menus ne sont pas à la même place, par exemple).

La console a toujours un fond noir et un texte blanc, je vous rassure (quoique ça se personnalise, ça). En revanche, les fonctionnalités offertes par l’invite de commandes peuvent varier en fonction du shell que l’on utilise.

Lire la suite…

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Bash: How do I get the command history in a screen session?

05/07/2017 Comments off

If I start a screen session with screen -dmS name, how would I access the command history of that screen session with a script?

Using the , the last executed command appears, even in screen.


I use the default bash shell on my system and so might not work with other shells.

this is what I have in my ~/.screenrc file so that each new screen window gets its own command history:

Default Screen Windows With Own Command History

To open a set of default screen windows, each with their own command history file, you could add the following to the ~/.screenrc file:

screen -t "window 0" 0 bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'
screen -t "window 1" 1 bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'
screen -t "window 2" bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'

Ensure New Windows Get Their Own Command History

The default screen settings mean that you create a new window using Ctrl+a c or Ctrl+a Ctrl+c. However, with just the above in your ~/.screenrc file, these will use the default ~/.bash_history file. To fix this, we will overwrite the key bindings for creating new windows. Add this to your ~/.screenrc file:

bind c screen bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'
bind ^C screen bash -ic 'HISTFILE=~/.bash_history.${WINDOW} bash'

Now whenever you create a new screen window, it’s actually launching a bash shell, setting the HISTFILE environmental variable to something that includes the current screen window’s number ($WINDOW).

Command history files will be shared between screen sessions with the same window numbers.

Write Commands to $HISTFILE on Execution

As is normal bash behavior, the history is only written to the $HISTFILE file by upon exiting the shell/screen window. However, if you want commands to be written to the history files after the command is executed, and thus available immediately to other screen sessions with the same window number, you could add something like this to your ~/.bashrc file:

export PROMPT_COMMAND="history -a; history -c; history -r; ${PROMPT_COMMAND}"
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Learn Bash: Remove Commands From Your History

21/06/2017 Comments off

Occasionally I type a password or other sensitive information into a shell prompt. Using bash history, the command can be removed.

# say we start with an empty bash command history
bash-3.2$ history
 1 history
# enter a command that requires a password
bash-3.2$ sudo rm -i some_file
# accidentally ^C and type your password
# into the prompt and hit enter
bash-3.2$ secret_password
bash: secret_password: command not found
# your password is now there for all to
# see in your bash history
bash-3.2$ history
 1 history
 2 sudo rm -i some_file
 3 secret_password
 4 history
# first option to fix it, delete the numbered entry from
# history and write to your ~/.bash_history file
bash-3.2$ history -d 3
bash-3.2$ history -w
# entry 3 will be removed entirely from your command history
bash-3.2$ history
 1 history
 2 sudo rm -i some_file
 3 history
 4 history -d 3
 5 history -w
 6 history
# the second option is to clear the entire history
# and write the changes to disk
bash-3.2$ history -c
bash-3.2$ history -w
# it's now pretty obvious that your history has been
# scrubbed clean, but at least your password is history!
bash-3.2$ history
 1 history -w
 2 history
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Dumper une base MySQL avec horodatage dans le nom du fichier

03/04/2017 Comments off

J’ai un projet qui utilise la base de données MySQL. Je souhaite sauvegarder la base de données tous les jours, donc j’utilise ceci:

mysqldump -h host -u user -p database de mots de passe> 'location.sql'

Je souhaite que les fichiers soient nommés avec l’horodatage, c’est-à-dire:

Aujourd’hui, le fichier sera nommé quelque chose-07-05-2014 08-00-00
Demain sera quelque chose-08-05-2014 08-00-00

Comment ajouter un timestamp formaté avec le nom de fichier exporté?


Une solution serait:

mysqldump -h host -u user -p password database > quelque chose-$(date +%d-%m-%Y %H %M %S).sql

Pour un horodatée qui permette le tri correct des fichiers, il fait changer l’ordre des paramètres et utiliser:


de manière à trier sur année, mois puis jour. Ne rien changer pour les hh:mm:ss puisque le tri se fair naturellement dans cas.

Pour automatiser ce dump, il faut insérer cette commande dans le crontab (du root ou de l’utilisateur):

0 */8 * * * root /usr/bin/mysqldump -h host -u user -p password database > quelque chose-$(date +%d-%m-%Y %H %M %S).sql

pour que la commande s’exécute toutes les 3 heures (24h ÷ 8).

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