Set or Change System Hostname

This will return the current hostname

$ hostname
MyHostame

To set hostname immediately:

$ hostname [MY_HOSTNAME]

This will change the hostname of your system immediately, but the original hostname will be restored upon next reboot. To set hostname to be permant follow the instructions below. For systemd based distributions(newer version of different Linux distributions such as latest Ubuntu, Debian, CentOS, Fedora, RedHat, etc. ) use the hostnamectl command:

$ sudo hostnamectl set-hostname [MY_HOSTNAME]

For sysvinit(in short init) based distributions(older linux distributions) edit /etc/hostname:

# vim /etc/hostname

Add a new line for the hostname in /etc/hosts:

# vim /etc/hosts

127.0.0.1 [MY_HOSTNAME]

And run:

# /etc/init.d/hostname restart

For RHEL/CentOS based systems that use init, edit /etc/sysconfig/network:

# vi /etc/sysconfig/network

/etc/sysconfig/network
NETWORKING=yes
HOSTNAME="[MY_HOSTNAME]"
GATEWAY="192.168.0.1"
GATEWAYDEV="eth0"
FORWARD_IPV4="yes"

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shmget errno 22 EINVAL: "Invalid Argument"

The man page says:

EINVAL A new segment was to be created and size < SHMMIN or size > SHMMAX, or no new segment was to be created, a segment with given key existed, but size is greater than the size of that segment.

This means that a memory segment still persists in the system. If your process is no longer running, run the ipcs command and remove the segment with ipcrm.

[root@myhost]# ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x74010241 2818048    root      600        4          0
0x7401026c 2162689    root      600        4          0
0x00000000 3178498    root      644        52         2
0x74010240 2785283    root      600        4          0
0x00000000 3211268    root      644        16384      2
0x00000000 3244037    root      644        268        2
0x750ca262 3440646    root      644        2043293648 0

------ Semaphore Arrays --------
key        semid      owner      perms      nsems
0x000000a7 0          root      600        1

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages
0x7504d462 9568256    root       666        0            0
0x7504d562 9601025    root       666        0            0
0x7504d662 2686978    root       666        0            0
0x7504d762 2719747    root       666        0            0
0x7504d862 5373956    root       666        0            0
0x7504d962 5406725    root       666        0            0
0x75108a62 9502726    root       666        0            0
0x000002bd 9535495    root       666        0            0

[root@myhost]# ipcrm -m 3440646
[root@myhost]# ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x74010241 2818048    root      600        4          0
0x7401026c 2162689    root      600        4          0
0x00000000 3178498    root      644        52         2
0x74010240 2785283    root      600        4          0
0x00000000 3211268    root      644        16384      2
0x00000000 3244037    root      644        268        2

------ Semaphore Arrays --------
key        semid      owner      perms      nsems
0x000000a7 0          root      600        1

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages
0x7504d462 9568256    root       666        0            0
0x7504d562 9601025    root       666        0            0
0x7504d662 2686978    root       666        0            0
0x7504d762 2719747    root       666        0            0
0x7504d862 5373956    root       666        0            0
0x7504d962 5406725    root       666        0            0
0x75108a62 9502726    root       666        0            0
0x000002bd 9535495    root       666        0            0

[root@myhost]#

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tar with multiple directories and file locations

tar -czvf mytargz.tar.gz /path1/to/dir1 /path2/to/dir2 /path3/to/dir3

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Mocking a Web Service with NetCat

Write the web service's response(xml, json, plain text) in a file, and then run the following in a terminal:

$ while true; do echo -e "HTTP/1.1 200 OK\r\n\r\n$(cat wsresponse.file)" | nc -l 9999; done

Where wsresponse.file is the file with the web service response.

In this case, the web service mock is listening in the port 9999. Just change the port as your needs.

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Tcpdump Packet Capture Truncated

Older versions of tcpdump truncate packets to 68 or 96 bytes. To solve this, use -s 65535 to capture full-sized packet:

$ tcpdump -i [interface] -s 65535 -w [file]

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Find text inside compressed files

Designed by Freepik

You have a group of files and some of them are compressed with zip or gz and you want to search for a specific text in each of them. To do this, you can use the following command:

find -name [FILE_PATTERN] -print0 | xargs -0 zgrep "[TEXT_TO_FIND]"

Where:
[FILE_PATTERN] : is the pattern of the files.
[TEXT_TO_FIND] : is the text to find inside the files.

Let's say we have the following files:

peter-01.gz
peter-02
peter-03.gz
peterA
peterB.gz
maria-01
maria-02.gz

And you want to find the text "my friend" only in the files that begininng with "peter-". To do this, you can use the following:

find -name peter-\* -print0 | xargs -0 zgrep "my friend"

Note the use of  \* as a wildcard.

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Copy directories with scp

scp -rp [directory_to_copy] [user]@[host]:[path]

Where:

[directory_to_copy]: directory to be copied
[user]: destination host user
[host]: destination host
[path]: destination path
-r: recursive, copy all directory content.
-p: preserves modification times, access times, and modes from the original file.

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