What is Interpreter and Complier

We generally write a computer program using a high-level language. A high-level language is one which is understandable by us humans. It contains words and phrases from the English (or other) language. But a computer does not understand high-level language. It only understands program written in 0's and 1's in binary, called the machine code. A program written in high-level language is called a source code. We need to convert the source code into machine code and this is accomplished by compilers and interpreters. Hence, a compiler or an interpreter is a program that converts program written in high-level language into machine code understood by the computer.

The difference between an interpreter and a compiler is given below:

Interpreter	Compiler
Translates program one statement at a time.	Scans the entire program and translates it as a whole into machine code.
It takes less amount of time to analyze the source code but the overall execution time is slower.	It takes large amount of time to analyze the source code but the overall execution time is comparatively faster.
No intermediate object code is generated, hence are memory efficient.	Generates intermediate object code which further requires linking, hence requires more memory.
Continues translating the program until the first error is met, in which case it stops. Hence debugging is easy.	It generates the error message only after scanning the whole program. Hence debugging is comparatively hard.
Programming language like Python, Ruby use interpreters.	Programming language like C, C++ use compilers.

 

How to set Quota in Linux

What is Quota

Disk space can be restricted by implementing disk quotas which alert a system administrator before a user consumes too much disk space or a partition becomes full.

Disk quotas can be configured for individual users as well as user groups. This makes it possible to manage the space allocated for user-specific files (such as email) separately from the space allocated to the projects a user works on (assuming the projects are given their own groups).

In addition, quotas can be set not just to control the number of disk blocks consumed but to control the number of inodes (data structures that contain information about files in UNIX file systems). Because inodes are used to contain file-related information, this allows control over the number of files that can be created.

Prerequisite : The quota RPM must be installed to implement disk quotas.

How to Enable

To implement disk quotas, use the following steps:

Enable quotas per file system by modifying the /etc/fstab file.

Remount the file system(s).

Create the quota database files and generate the disk usage table.

Assign quota policies.

Each of these steps is discussed in detail in the following sections.

Enabling Quotas

As root, using a text editor, edit the /etc/fstab file.

Add the usrquota and/or grpquota options to the file systems that require quotas:

Example 16.2. Add quotas

/dev/VolGroup00/LogVol02 /home     ext3    defaults,usrquota,grpquota  1 2 

/dev/VolGroup00/LogVol01 swap      swap    defaults        0 0 . . .

In this example, the /home file system has both user and group quotas enabled.

Note

The following examples assume that a separate /home partition was created during the installation of Red Hat Enterprise Linux. The root (/) partition can be used for setting quota policies in the /etc/fstab file.

Remounting the File Systems

After adding the usrquota and/or grpquota options, remount each file system whose fstab entry has been modified. If the file system is not in use the following commands:

umount /mount-point

For example, umount /work.

mount /file-system /mount-point

For example, mount /dev/vdb1 /work.

If the file system is currently in use, the easiest method for remounting the file system is to reboot the system.

Creating the Quota Database Files

After each quota-enabled file system is remounted run the quotacheck command.

The quotacheck command examines quota-enabled file systems and builds a table of the current disk usage per file system. The table is then used to update the operating system's copy of disk usage. In addition, the file system's disk quota files are updated.

To create the quota files (aquota.user and aquota.group) on the file system, use the -c option of the quotacheck command.

Create quota files

For example, if user and group quotas are enabled for the /home file system, create the files in the /home directory:

# quotacheck -cug /home

The -c option specifies that the quota files should be created for each file system with quotas enabled, the -u option specifies to check for user quotas, and the -g option specifies to check for group quotas.

If neither the -u or -g options are specified, only the user quota file is created. If only -g is specified, only the group quota file is created.

After the files are created, run the following command to generate the table of current disk usage per file system with quotas enabled:

# quotacheck -avug

The options used are as follows:

a

Check all quota-enabled, locally-mounted file systems

v

Display verbose status information as the quota check proceeds

u

Check user disk quota information

g

Check group disk quota information

After quotacheck has finished running, the quota files corresponding to the enabled quotas (user and/or group) are populated with data for each quota-enabled locally-mounted file system such as /home.

Assigning Quotas per User

The last step is assigning the disk quotas with the edquota command.

To configure the quota for a user, as root in a shell prompt, execute the command:

# edquota username

Perform this step for each user who needs a quota. For example, if a quota is enabled in /etc/fstab for the /home partition (/dev/VolGroup00/LogVol02 in the example below) and the command edquota testuser is executed, the following is shown in the editor configured as the default for the system:

Disk quotas for user testuser (uid 501):   

Filesystem                blocks     soft     hard    inodes   soft   hard   

/dev/VolGroup00/LogVol02  440436        0        0     37418      0      0

Note : The text editor defined by the EDITOR environment variable is used by edquota. To change the editor, set the EDITOR environment variable in your ~/.bash_profile file to the full path of the editor of your choice.

The first column is the name of the file system that has a quota enabled for it. The second column shows how many blocks the user is currently using. The next two columns are used to set soft and hard block limits for the user on the file system. The inodes column shows how many inodes the user is currently using. The last two columns are used to set the soft and hard inode limits for the user on the file system.

The hard block limit is the absolute maximum amount of disk space that a user or group can use. Once this limit is reached, no further disk space can be used.

The soft block limit defines the maximum amount of disk space that can be used. However, unlike the hard limit, the soft limit can be exceeded for a certain amount of time. That time is known as the grace period. The grace period can be expressed in seconds, minutes, hours, days, weeks, or months.

If any of the values are set to 0, that limit is not set. In the text editor, change the desired limits.

Change desired limits

For example:

Disk quotas for user testuser (uid 501):   

Filesystem                blocks     soft     hard   inodes   soft   hard   

/dev/VolGroup00/LogVol02  440436   500000   550000    37418      0      0

To verify that the quota for the user has been set, use the command:

# quota username

Disk quotas for user username (uid 501): 

   Filesystem  blocks   quota   limit   grace   files   quota   limit   grace

     /dev/sdb    1000*   1000    1000               0       0       0

Assigning Quotas per Group

Quotas can also be assigned on a per-group basis. For example, to set a group quota for the devel group (the group must exist prior to setting the group quota), use the command:

# edquota -g devel

This command displays the existing quota for the group in the text editor:

Disk quotas for group devel (gid 505):   

Filesystem                blocks    soft     hard    inodes    soft    hard   

/dev/VolGroup00/LogVol02  440400       0        0     37418       0       0

Modify the limits, then save the file.

To verify that the group quota has been set, use the command:

# quota -g devel

Setting the Grace Period for Soft Limits

If a given quota has soft limits, you can edit the grace period (i.e. the amount of time a soft limit can be exceeded) with the following command:

# edquota -t

This command works on quotas for inodes or blocks, for either users or groups.

Important

While other edquota commands operate on quotas for a particular user or group, the -t option operates on every file system with quotas enabled.