The flag looks almost like the japanese flag, only the color is different.

The bangladeshi flag is a red disc on green field.The background green colour symbolizes the greenery of Bangladesh while the red disc represents the rising sun and the sacrifice Bangladeshi people made to obtain their independence from pakistan.It is also said that the red circular disc symbolizes blood of thousands of Bangladeshis killed by the Pakistanis since 1947. The green backdrop, needless to say,symbolizes the greenery of the land.However according to the bangladeshis of islamist bent, the green colour symbolises islam.But the unique feature of the flag was absence of the crescent and star (as in the Pakistanīs flag),symbol of the Islamic states. Sheikh Mujibur Rahman,the father of the nation and one who got independence of Bangladesh from pakistan dreamed for the secular Bangladesh.Unfortunetely it remained only a dream.Just like what happens in many muslim countries,he was overthrown and killed by an islamist general and the country was declared an islamic republic.
This official flag was first flown by the Bengali freedom fighters on top of the Pakistani embbasy in Kolkatta(now Bangladeshi embassy).

Pakistani embbasy in Kolkatta

A slight calrifacation, that would be the Pakistani consulate in what was then Calcutta.

The Pakistani Embassy in India is in New Delhi and has been there since 1947.

The current Bangladeshi Embassy is also in New Delhi so the former Pakistani consulate in question has now become the Bangladeshi consulate in Kolkatta.

"Who are the rulers of the lands beyond Guraiz and Burzil? The Darada Rajas."
The Rajatarangini

infact crescent and star are not related with islam, Turks had flags with crescent and star long before islam. There are tombs in central asia with crescent and star figure from BC400s. But worldwide public sentiment was "islam = turks" in the ottoman age.

Kardesh Khun,

Their is no crescent on the Bangladeshi flag, but their is one on the Pakistani one.

The Turks may have used the crescent as a symbol before Islam, but the crescent was also a symbol used by the earliest Arab Muslims, and the Holy Prophet himself.

Under the Ottomans their were two flags, one was the current Turkish flag, which was the political flag of the Ottoman Empire and the other one was a green flag with three crescents, that was the flag of the Caliph and it was the same flag that was used by the Holy Prophet Muhammad SAWS.

So for secular purposes the Sultan used the Turkish flag and in his capacity as Caliph he used the Islamic one.

"Who are the rulers of the lands beyond Guraiz and Burzil? The Darada Rajas."
The Rajatarangini