According to the official project site:
netfilter is a set of hooks inside the Linux kernel that allows kernel modules to register callback functions with the network stack. A registered callback function is then called back for every packet that traverses the respective hook within the network stack.
This Linux based firewall is controlled by the program called iptables to handles filtering for IPv4, and ip6tables handles filtering for IPv6. I strongly recommend that you first read our quick tutorial that explains how to configure a host-based firewall called Netfilter (iptables) under CentOS / RHEL / Fedora / Redhat Enterprise Linux. This post lists most simple iptables solutions required by a new Linux user to secure his or her Linux operating system from intruders. Lire la suite…
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.
Lire la suite…
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 $1}'
Sample outputs:
5253
OR assign this size to a bash variable:
myfilesize=$(wc -c "/etc/passwd" | awk '{print $1}')
printf "%d\n" $myfilesize
echo "$myfilesize"
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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"
OR
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" $myFileSizeCheck
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The syntax is as follows to get the file size on BSD/MacOS stat:
stat -f %z "/etc/passwd"
Please note that if the file is symlink you will get size of that link only with the stat command.
du command example
The syntax is
du --apparent-size --block-size=1 "/etc/passwd"
fileName="/etc/hosts"
mfs=$(du --apparent-size --block-size=1 "$fileName" | awk '{ print $1}')
echo "$fileName size = ${mfs}"
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Sample outputs from above commands:
Fig.01: How to check size of a file using a bash/ksh/zsh/sh/tcsh shell?
Find command example
The syntax is:
find "/etc/passwd" -printf "%s"
find "/etc/passwd" -printf "%s\n"
fileName="/etc/hosts"
mysize=$(find "$fileName" -printf "%s")
printf "File %s size = %d\n" $fileName $mysize
echo "${fileName} size is ${mysize} bytes."
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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:
3
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
OR
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 command
I type ‘find . -type d -iname foo -delete‘ command to find the foo directories and delete them. However, I am getting an error message that read as find: cannot delete './hourly.4/data/foo': Directory not empty on Linux server. How do delete directories based on find command output on Linux or Unix-like system?
The -delete option remove the DIRECTORY(ies), if they are empty. You need to use the -execoption to delete all directories and its contents. The syntax is as follows.
Find command syntax to delete dirs
Try:
find /dir/to/search/ -type d -name "dirName" -exec rm -rf {} +
OR
find /dir/to/search/ -type d -name "dirName" -exec rm -rf \;
Warning: Be careful with the rm command when using with find. You may end up deleting unwanted data.
Find will execute given command when it finds files or dirs. For example:
find . -type d -name "foo" -exec rm -rf {} +
OR
find . -type d -name "bar" -exec rm -rf "{}" \;
Sample outputs:
removed './daily.0/bar/.cache/motd.legal-displayed'
removed directory './daily.0/bar/root/.cache'
removed './daily.0/bar/.lesshst'
removed './daily.0/bar/.viminfo'
removed './daily.0/bar/.vim/.netrwhist'
removed directory './daily.0/bar/root/.vim'
removed './daily.0/bar/root/.bashrc'
removed './daily.0/bar/.ssh/authorized_keys'
removed directory './daily.0/bar/root'
removed directory './daily.0/bar/var/spool/cron/crontabs'
You can find directories that are at least four levels deep in the working directory /backups/:
find /backups/ -type d -name "bar" -depth +4 -print0 -exec rm -rf {} +
Find and xargs
The syntax is as follows to find and delete directories on Linux/Unix system:
## delete all empty dirs ##
find /path/to/dir/ -type d -empty -print0 | xargs -0 -I {} /bin/rm -rf "{}"
## delete all foo dirs including subdirs in /backups/
find /backups/ -type d -name "foo*" -print0 | xargs -0 -I {} /bin/rm -rf "{}"
The second command is secure and fast version as it deals with weird dir names such as:
