Bash - Bourne Again Shell
File descriptors and redirections
# <<: # This is known as a "here document" or "here-strings" operator. It allows you to input multiple lines of text into a command or a file. Example:
cat << EOF > stream.txt
1. cat: It is a command used to display the contents of a file.
2. <<: This is known as a "here document" or "here-strings" operator. It allows you to input multiple lines of text into a command or a file.
3. EOF: It stands for "End of File" and serves as a delimiter to mark the end of the input. You can choose any other unique string instead of "EOF" as long as it is consistent with the opening and closing delimiter.
4. >: This is a redirection operator used to redirect the output of a command to a file. In this case, it will create a new file named stream.txt or overwrite its contents if it already exists.
5. stream.txt: It is the name of the file where the input text will be written.
Shortcuts
+Delete last word CTRL-W +Fill out the next word that appears in gray: CTRL ->
Commands
df
Displays the amount of space available on the file system containing each file name argument.
host
host is a simple utility for performing DNS lookups. It is normally used to convert names to IP addresses and vice versa. When no arguments or options are given, host prints a short summary of its command-line arguments and options.
# General syntax
host <name> <server>
# <name> is the domain name that is to be looked up. It can also be a dotted-decimal IPv4 address or a colon-delimited IPv6 address, in which case host by default performs a reverse lookup for that address.
# <server> is an optional argument which is either the name or IP address of the name server that host should query instead of the server or servers listed in /etc/resolv.conf.
Example:
lsblk
lsusb
lsof
lspci
lsb_release
Print distribution-specific information
netstat
Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships. By default, netstat displays a list of open sockets. If you don't specify any address families, then the active sockets of all configured address families will be printed.
netstat -tnlp
# -p: Show the PID and name of the program to which each socket belongs.
# -l: Show only listening sockets.
# Show networks accessible via VP
netstat -rn
# -r: Display the kernel routing tables. Replacement for netstat -r is "ip route".
# -n: Show numerical addresses instead of trying to determine symbolic host, port or user names.
sed
sed looks for patterns we have defined in the form of regular expressions (regex) and replaces them with another pattern that we have also defined. Let us stick to the last results and say we want to replace the word "bin" with "HTB."
The "s" flag at the beginning stands for the substitute command. Then we specify the pattern we want to replace. After the slash (/), we enter the pattern we want to use as a replacement in the third position. Finally, we use the "g" flag, which stands for replacing all matches.
ss
Sockets statistic. It can be used to check which ports are listening locally on a given machine.
ss -ltn
#-l: Display only listening sockets.
#-t: Display TCP sockets.
#-n: Do not try to resolve service name.
How many services are listening on the target system on all interfaces? (Not on localhost and IPv4 only):
ss -l -4 | grep -v "127\.0\.0" | grep "LISTEN" | wc -l
# **-l**: show only listening services
# **-4**: show only ipv4
# **-grep -v "127.0.0"**: exclude all localhost results
# **-grep "LISTEN"**: better filtering only listening services
# **wc -l**: count results
uname
# Print out the kernel release to search for potential kernel exploits quickly.
uname -r
```shell-session
## Flags
-a, --all
print all information, in the following order, except omit -p and -i if unknown:
-s, --kernel-name
print the kernel name
-n, --nodename
print the network node hostname
-r, --kernel-release
print the kernel release
-v, --kernel-version
print the kernel version
-m, --machine
print the machine hardware name
-p, --processor
print the processor type (non-portable)
-i, --hardware-platform
print the hardware platform (non-portable)
-o, --operating-system