Monitor network traffic with tcpdump

Catch all traffic between the hosts host1.example.org and host2.example.org on all interfaces and dump the traffic to the file dump2.raw

tcpdump host host1.example.org and host2.example.org -i any -w dump2.raw

Using tar

compress tar

 tar cvf mytar.tar . 

extract tar

 tar xvf mytar.tar 

Only show the contents of tar archive:

 tar tvf mytar.tar 

install and uninstall rpm packages under Linux

Install a given rpm package. –force overwrites the package if it is already installed

rpm -i --force myrpm.rpm 

List installed rpm packages (-qa: query all)

rpm -qa

Find the exact name of an installed rpm package

rpm -qa | grep java

Uninstall the Java package in one step by using a subcommand.

rpm -e `rpm -qa | grep $PKG`

uninstall package without using scripts. This is useful if the package won’t uninstall when trying to uninstall it normally. Afterwards the package is not listed anymore by rpm -qa

rpm -e mypackage --noscripts

secure copy (scp) between two UNIX machines

Copy a directory from localhost to a remote directory on a remote machine:

scp -r mydir username@remotemachine:/home/remotedirectory/

Copy a remote directory from a remote machine to the current directory:

scp -r username@remotemachine:/home/remotedirectory/ .

username is the user on remote host
remotemachine is the remote machine’s name or IP
remote directory is specified after the colon

execute subcommand in UNIX with `command`

A subcommand can be exceuted when enclosing it in single backticks (`):
Examples:

mv a `echo b`

List all packages (-qa) containing the name under $PKG and deinstall it (-e option) if only one is returned.

rpm -e `rpm -qa | grep $PKG`

Delete user and group under Linux

/usr/sbin/userdel myuser
/usr/sbin/groupdel mygroup 

Redirect standard output (stdout) and standard error (stderr) to a single file under linux

 command > file 2>&1 

ex.:

 ps -e > log.txt 2>&1 

Convert Windows files to Unix by removing carriage returns (batch, bulk)

If you edit files under Windows and then move them to a Linux server, you may encounter the problem that Windows uses \r\n as newline while Linux just uses \n. To remove the carriage return in all files ending on .sh or .SPEC use the following command:

 find . -type f \( -name "*.sh" -o -name "*.SPEC" \) -exec sh -c "tr -d '\15\32' {} > {}.nn" \; 

For each file found by find, a shell is executed, which again executes the tr command to drop \r. The tr command is given the current file via the {} parameter. The output of the tr command is redirected from the standard output to a new file that consists of the current file’s name ({} again} plus the ending .nn.

rename file endings throughout subdirectories in Linux

Remove the file ending .nn in this directory and all subdirectories:

rename .nn "" **/*.nn

RPM Package unter Red Hat Enterprise Linux entpacken

rpm2cpio my_pkg.rpm | cpio -dimv