TIL: Very useful Linux/Unix commands

Here is a list of useful unix commands or code parts. Who does not know it? You have a problem and looking for a solution where you find at stack overflow or similar pages? Here I collect all the commands that I have encountered over time or whose switch I simply can not remember (or want).

  • How do I find all files containing specific text?
grep -rnw '/path/to/somewhere/' -e 'pattern'
  • How i change the default file permissions (mask that controls file permissions)
  • Untar (unzip) file/folder
tar -zxvf archive.tar.gz
  • Tar (zip) file/folders
tar -cvzf archive.tar.gz file1 file2
  • Copy files via rsync from one host to another
rsync -avz [USER@]HOST:SOURCE rsync://[USER@]HOST[:PORT]/DEST
rsync -avz -e "ssh -p 12345" LOCAL/SOURCE [USER@]HOST:DEST
  • nslookup missing? Install dig
sudo apt-get install dnsutils
  • find without “Permission denied”
find / -name 'filename.ext' 2>&1 | grep -v "Permission denied"
  • flush dns cache
sudo systemd-resolve --flush-caches
  • show open ports
netstat -tulpn
  • Directory size
du -sh /var
du -shc /var/*
du -h --max-depth=1 /var
du -sh /var/lib/docker/containers/*/*.log
  • Search multiple PDF files for a “needle”
pdfgrep -i needle haystack*.pdf
  • Show hidden files with ls
ls -lar
  • Redirect STDOUT and STDERR to a file
nice-command > out.txt 2>&1
  • Installs your SSH public key to a remote host
sh-copy-id 'user@remotehost'
  • A command-line system information tool
  • Show disk usage, folder size, items per folder, find big directorys, … with ncdu

Run command in background on a Synology NAS with nohup

To run a programm¬† that doesn’t quit if you close the ssh session use nohup (no hangup).¬†Attention, you have to run it as root for it to work!

admin@DiskStation:~$ sudo su
ash-4.3# nohup <command> &

ash-4.3# nohup cat /dev/zero | split -b 4095m - /volumeUSB2/usbshare/zeros -d --additional-suffix=.file &

PowerCLI error after vCenter Server upgrade

After upgrading to vSphere 6.7, a PowerCLI script aborts with this error message:

The vCenter Server is unable to decrypt passwords stored in the customization specification.

To resolve the issue, retype the password in the VMcustomization specifications (under Policies and Profiles). Edit the customization specifications and retype the password under the following two preference points:
– Administrator password
– Workgroup or domain

Write zeros to a hard drive – Wipe/Erase unused or free space

Windows: Format and write zeros to every sector of the drive.
format <Driveletter> /fs:NTFS /p:0

Windows: Format and write zeros to every sector of the drive. After the first pass write random numbers.
format <Driveletter> /fs:NTFS /p:2

Windows: Fill free space.
cipher /w:<Driveletter>[:\foldername]</span>

Unix: Wipe full drive.
dd if=/dev/zero of=/dev/<block device> bs=<block size 1M, 32M, etc.> status=progress

Unix: Fill free space.
dd if=/dev/zero of=/path/to/drive/zeros.file status=progress

Unix: Fill free space on FAT32 drive (4GB limit per file).
cat /dev/zero | split -b 2000m - zero -d --additional-suffix=.file

To run the process in background see Run command in background on a Synology NAS with nohup

How to configure RPC dynamic port allocation to work with firewalls

Windows XP use per default a dynamic port range from 1024 to 5000 for RPC/WMI/DCOM. To set up a fixed Port or Range use this settings:

  1. regedit: HKEY_LOCAL_MACHINE\Software\Microsoft\Rpc
  2. Add key “Internet”
  3. Add this three values:

Ports: REG_MULTI_SZ: 5000-5100
PortsInternetAvailable: REG_SZ: Y
UseInternetPorts: REG_SZ: Y

  1. run winmgmt -standalonehost
  2. net stop winmgmt
  3. net start winmgmt
  4. Reboot