How can I find some text in the files of this directory?

Well most of us used to use grep in a pipe with other commands like netstat, ss, ps, etc., but grep is a very powerful tool and for instance could help you to locate text in the files of a directory.
Use:

$ grep -iR "text-to-find" *

The parameter -i is for no case sensitive search and -R is for recursive search.
As a result you can find a list of your files that contains the text and the line containing it.

See and example:

$ cd $TOMCAT_HOME
$ grep -iR "org.apache.tomcat" *
...
conf/catalina.properties:org.apache.naming.resources.,org.apache.tomcat.
conf/catalina.properties:org.apache.jasper.,org.apache.naming.,org.apache.tomcat.
...

The grep command syntax is the following:

grep [OPTIONS] PATTERN [FILE...]

Where the options are:

-c : This prints only a count of the lines that match a pattern
-h : Display the matched lines, but do not display the filenames.
-i : Ignores, case for matching
-l : Displays list of a filenames only.
-n : Display the matched lines and their line numbers.
-v : This prints out all the lines that do not matches the pattern
-e exp : Specifies expression with this option. Can use multiple times.
-f file : Takes patterns from file, one per line.
-E : Treats pattern as an extended regular expression (ERE)
-w : Match whole word
-o : Print only the matched parts of a matching line,

Getting system info

Sometime you probably need to know some details for your systems as the linux distribution and version, type of CPU, etc.
These are some commands that could help you.

$ hostnamectl
   Static hostname: YOURHOSTNAME
         Icon name: computer-vm
           Chassis: vm
        Machine ID: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
           Boot ID: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
    Virtualization: vmware
  Operating System: Ubuntu 16.04.5 LTS
            Kernel: Linux 4.4.0-130-generic
      Architecture: x86-64

$ uname -a

Linux YOURHOSTNAME 4.4.0-130-generic #156-Ubuntu SMP Thu Jun 14 08:53:28 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

If you need hardware detailed info try with lshw:

$ lshw -short

H/W path     Device  Class      Description

===========================================

                     system     Computer

/0                   bus        Motherboard

/0/0                 memory     7974MiB System memory

/0/1                 processor  Intel(R) Xeon(R) CPU E5-2670 v3 @ 2.30GHz

...

If you need OS information, you have it on /etc/os-release.

$ cat /etc/os-release
NAME="Oracle Linux Server"
VERSION="7.5"
...

You can find more info on this link https://www.cyberciti.biz/faq/how-to-check-os-version-in-linux-command-line/.

Enjoy!

Identify a process who is listening in a port

Sometimes you need to identify a process than is associated to a TCP port.

You can use ss instead netstat command to identify them.

For instance, if you want to identify what is the process who is listen on TCP port 80, use the following command:

# ss -ltnp | grep -w ':80'

LISTEN   0         128                 0.0.0.0:80               0.0.0.0:*        users:(("nginx",pid=24503,fd=13),("nginx",pid=24502,fd=13),("nginx",pid=24499,fd=13))

Where:

• l – tells netstat to only show listening sockets.
• t – tells it to display tcp connections.
• n – instructs it show numerical addresses.
• p – enables showing of the process ID and the process name.

Then, with the process number (i.e. 24499) you can get the process detail with:

# ls -l /proc/24499/exe
lrwxrwxrwx. 1 root root 0 Jun  3 08:37 /proc/24499/exe -> /opt/opscode/embedded/sbin/nginx