Unix tips | Part 1

I would like to share in this series of posts all the interesting tips in UNIX i’ve came across till date.

To understand read, write, execute permissions for a file
```
ls -l file.dat
-rw-r--r-- 1 root users 1656 Mar 22 00:27 file.dat
```
The first letter – stands for a normal file, d for directory, c or b for a device, l for a link to another file. Then comes read write execute permissions for user, group and others respectively.
Repeat last command !!. Example say you want to install vim in ubuntu. apt-get install vim this will fail because you do not have root permissions sudo !! will install it.
To check if a given command executed successfully or not, type the following command.
```
echo $?
```
Zero (0) result represents success and any Non-zero result represents failure.
To view latest output appended to a file as it grows
```
tail -f path/to/file
```
To get name server information for domains by querying DNS
```
nslookup -type=ALL domain.name
```
Two or more commands can be combined with the && operator. However the succeeding command is executed if and only if the previous one is true. If you want to execute both irrespective of first outcome use & instead.
```
ls && date
# lists the contents of the directory first and then gives the system date.
```
To get list of all the installed packages
```
dpkg -l
```

The colour code of the files is as follows.

Blue : Directory file
White : Normal file
Green : Executable file
Yellow : Device file
Magenta : Picture file
Cyan : Link file
Red : Compressed file

sandheepg