BASH BD

by Rafael Domiciano

Fast reading

Initialize database

./bd -b data

Open the database

./bd data

For help type ?

Long reading

BashBD. What is it?
Code .1 Functions prototypes .2 Returning values .3 Comment on the header of the function .4 About "source" or "dot" to import .5 "variable" vs "constants" .6 Handling Errors .7 Some conventions used .8 Calling commands .9 Logging .10 dump_data script
Structures .1 Code Workflows .2 File format
Metadata

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1. BashBD. What is it?

BashBD is a DBMS writeten entirely in bash. Once there are a huge number of DBMS out there, the question that may arise is: But why? I script in bash for a long time and I'm a Postgres DBA for so long too, so I asked myself if I could do this, as a challenge to myself. This program is not meant to be perfomatic. During this process I had learned a lot not only of bash, but also about the postgres internals (a lot of code readed).

Its important to say, of course, DO NOT use this program in a production environment.

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2. Code conventions

2.1 Functions Prototypes

Return code of the function Every function should have a return code, using the return command Doing this the caller could analyze the return with the $? variable just after the function. This is similar to this c function: int fnc() { return 0; }

Example:

#!/bin/bash
fnc() {
	if [ true ]; then
		return 0
	else
		return 1
	fi
}
fnc
echo $?

Aditionally functions should use local variable with "_" initiating the name. Example:

#!/bin/bash
# $1 - Input value
fnc() {
	local _INPUT=$1
}

2.2 Returning values

If the function must return a value, the first parameter of the function is used for this purpose. Is similar to an c function that has the first parameter returning by reference.

Example: #!/bin/bash

$1 - The return

fnc() { # Declaring the local variable and setting to variable outside of this function local _RET=$1

# Setting the variable, by reference. Must have the "eval" command
eval $_RET="Value inside the function"

}

Just pass the name of the variable, without the "$" symbol

fnc VALUE echo $VALUE ## << this is going to print "Value inside the function"

Note: There are some cases that is not compliant with this convention . These are, in majority, the firsts functions created.

Note 2: Prefer this method of returning values than returning with echo, because this way is more faster. Further read: xxxx

2.3 Comment on the header of the function

The function must have all the parameters declared, like this:

#!/bin/bash

: <<'FUNCTION' Description: A short description Parameters: $1 - Param1 $2 - Param2 FUNCTION concat() { local _RET=$1

eval $_RET="$2$3"

}

2.4 About "source" or "dot" to import

In bash, there is not difference between using "source" or "dot". In the bash manual: source filename A synonym for "." filename

Ref: http://www.gnu.org/software/bash/manual/bashref.html#Bourne-Shell-Builtins

2.5 "variable" vs "constants"

Simple like this:

Put your variables in the "var" file
Put your constants in the "const" file, with the "readonly" keyword

2.6 Handling Errors

Handling errors in bash at the first look seems hard or complex, but it shouldn't be. Lets see:

As every function must return a code, you can read "$?"
Some functions could set ERRCODE (declared in var file) to further analysis
Doing LOGGER with the FATAL Loglevel, finish the program

2.7 Some conventions used

Prefer $(..) over .. [http://mywiki.wooledge.org/BashFAQ/082]

2.8 Calling commands

Everytime a command is called, bash searchs $PATH for its location to execute. To perform better, bashbd searchs for those path commands, done by function->[find_binaries]. The "find_binaries" save the path command in a variable with the following pattern: PRG_[COMMAND]. Some examples:

seq => PRG_SEQ
printf => PRG_PRINTF
echo => PRG_ECHO _ $PRG_ECHO "Test"

2.9 Logging

To generate log use the log function, like this:

# Source
. log

some_function() {
	eval $LOGGER LOG "Message to log"
}

You can use these log Level:

FATAL This level finish the program with functions.finish 1
LOG
ERROR

2.10 dump_data script

Something there is need to debug the table, you can use the dump_data script: ./dump_data data/1

3. Structures

As bash is not a programatically language, but a bunch of separated commands, and we do a lot steps to glue it together, it doesn't give the best performance perspective; so I have to do some decisions to not lose performance.

Each table is a single file. The file has a magic code determining the kind, like TABLE, INDEX, so on. Each record is separated by a magic Code 1E Each field/attribute inside the record is separated by a magic code 1D The "header" of the record has some metadata: pos1: Record is alive 0 - No 1 - Yes